UK-Mauritius treaty on the Chagos Archipelago

Approximate read time: 30 minutes

1. Background

1.1 What is the Chagos Archipelago?

The Chagos Archipelago is a group of islands in the Indian Ocean, located approximately 5,799 miles south-east of the UK and 1,250 miles north-east of Mauritius.^[1]

Figure 1. Map of the Chagos Archipelago

The islands became British territory in 1814, when they were ceded to the UK along with Mauritius (which then included the Seychelles). In 1965, the islands were designated as the British Indian Ocean Territory (BIOT) through an order in council under the royal prerogative, covering all islands in the archipelago. This order also provided for the appointment of a commissioner for the territory, who was granted the power to make laws for the “peace, order and good government” of the territory. In return for the detachment of the islands, the UK provided Mauritius with a £3mn grant in 1965 and committed to return the islands when they were no longer required for defence purposes.

In 1966, the UK agreed to make the BIOT available for the establishment of a joint UK-US military base on the largest of the islands, Diego Garcia.^[2] The base, leased to the US, operates as a joint US-UK military facility. Many displaced residents were resettled in Mauritius and the Seychelles, while some were relocated to the UK in 2002. Laws made by the territory’s commissioner in 1971, under the direction of UK government ministers, made it unlawful to enter or remain in the BIOT without a permit and authorised the removal of any remaining residents. This legally formalised the relocation of the civilian population and prohibited their return. Although there are no official figures, estimates suggest that between 1,400 and 1,700 Chagossians were removed from the islands during this period. In 2002, following changes to British nationality law, many Chagossians holding British Overseas Territories citizenship automatically became British citizens.

Compensation for the displaced Chagossians was provided by the UK in stages.^[3] £650,000 was given to Mauritius for resettlement in the 1970s, followed by an additional £4mn in the 1980s, distributed to Mauritius and Chagossian representatives in the form of a trust fund.

1.2 Legal history between the UK and Mauritius

Since Mauritius gained independence from the UK in 1968, there has been a bilateral dispute between the UK and Mauritius regarding sovereignty of the archipelago, including Diego Garcia. In recent years, Mauritius has pursued this dispute through claims before international courts and tribunals.

In 2019, the International Court of Justice (ICJ) gave an advisory opinion on the UK’s sovereignty over the islands.^[4] It stated that:

• the process of decolonisation of Mauritius was not lawfully completed when Mauritius was granted independence in 1968
• the UK was “under an obligation to bring to an end its administration of the archipelago as rapidly as possible”
• all UN member states were under an obligation to cooperate with the UN to complete the decolonisation of Mauritius

This opinion was followed by UN general assembly resolution 73/295 in May 2019, which welcomed the ICJ’s advisory opinion and called on specialised agencies to recognise that the Chagos Archipelago “forms an integral part of the territory of Mauritius”.^[5] The resolution was passed with 116 votes in favour to six votes against, with 56 abstentions.

1.3 Summary of negotiations between the UK and Mauritian governments

In November 2022, the then foreign secretary, James Cleverley, published a written ministerial statement following a meeting between then UK prime minister Rishi Sunak and then Mauritian prime minister Pravind Jugnauth at the UN general assembly. The statement announced that both countries had agreed to begin negotiations on the exercise of sovereignty over the Chagos Archipelago.^[6] Mr Cleverley stated that the objective of the negotiations was to “resolve all outstanding issues, including those relating to the former inhabitants of the Chagos Archipelago”. Regarding the future use of Diego Garcia, he noted that the UK and Mauritius had reiterated that any agreement between the two countries “will ensure the continued effective operation of the joint UK/US military base on Diego Garcia”. The statement also expressed the shared aim of reaching an agreement “by early next year”.

Further written ministerial statements were published by the foreign secretary on 17 March and 14 September 2023, confirming that “constructive negotiations” had taken place and that both leaders had agreed to continue negotiations.^[7] The previous government oversaw 11 rounds of negotiations between November 2022 and June 2024. This was then followed by two additional rounds of negotiations held by the current government in August and September 2024.^[8]

On 3 October 2024, the UK and Mauritian governments published a joint statement announcing that a political agreement had been reached.^[9] The agreement would be subject to the finalisation of a treaty and supporting legal instruments.

The treaty was finalised through technical discussions held in Mauritius in December 2024 and London in January 2025.^[10] On 13 January 2025, the parties published another joint statement outlining that “good progress” had been made and that discussions were ongoing to reach an agreement “in both sides’ interests”.^[11]

The treaty was formally approved and signed by UK Prime Minister Sir Keir Starmer and Mauritian Prime Minister Navin Ramgoolam on 22 May 2025.^[12]

The UK government consulted the US government on the final treaty prior to its approval. The US government stated that, following a “comprehensive interagency review”, it had determined that the agreement “secures the long-term, stable, and effective operation of the joint US-UK military facility at Diego Garcia”.^[13] Despite initial concern within the US government about the agreement, President Trump said during a meeting with Sir Keir Starmer in February 2025 that he was “inclined” to go along with the agreement and that he had “a feeling that it’s going to work out very well”.^[14]

In a press conference about the signing of the treaty, Sir Keir Starmer described the agreement as “absolutely vital” for the country’s defence and intelligence, “and therefore for the safety and security of the British people”.^[15] Sir Keir outlined several reasons for the government’s decision to sign the agreement with Mauritius:

• The UK would “not have a realistic prospect of success” if Mauritius pursued legal action and could have faced a provisional measures order “within a matter of weeks”.
• Even if the UK disregarded legal judgments against it, “international organisations and other countries would act on them”, which would “undermine the operation of the base”. The prime minister warned that without an agreement the UK would not be able to prevent China or any other nations from establishing bases on the outer islands or conducting joint exercises near Diego Garcia.
• The base provides the UK and US with “access to unique and vital capabilities”, including airfield and deep-water port facilities, infrastructure supporting global GPS operations and the monitoring of objects in Earth’s orbit, and equipment to monitor the nuclear test ban treaty.

In a statement in the House of Commons on the same day, Defence Secretary John Healey further outlined why the government had signed the agreement.^[16] He explained:

Agreeing this treaty now on our terms means that the UK retains full control over Diego Garcia now and for the next century […] At a cost of less than 0.2 percent of the annual defence budget we have secured unrestricted access to, and use of, the base, as well as control over movement of all persons and all goods on the base and control of all communication and electronic systems. Nothing can be built within a 24 nautical-mile buffer zone without our say so, and we have secured an effective veto on all developments in the Chagos archipelago, and a strict ban will be imposed on foreign security forces operating on the outer islands.^[17]

Additionally, Mauritian Prime Minister Navin Ramgoolan described the agreement as a “great victory”.^[18] He stated:

We have gained recognition of our sovereignty over the entire archipelago of Chagos, including Diego Garcia, which completes the process of decolonisation which began in 1968.

2. What are the terms of the agreement?

The agreement transfers sovereignty of the islands back to Mauritius, while the UK maintains an initial 99-year lease of Diego Garcia (with an option of extension). Additionally, it includes provisions on cooperation on environmental protection, maritime security and creating a trust fund for the benefit of Chagossians. The agreement’s documents include 19 articles, two annexes, and exchanges of letters between the UK and Mauritian governments.

2.1 Mauritian sovereignty over the Chagos Archipelago, including Diego Garcia

In the preamble of the agreement, the UK and Mauritian governments stated they recognised the “long and shared history” between the countries and expressed a commitment to “build a close and enduring bilateral partnership based on mutual respect and trust that enhances the prosperity and well-being of our people, the rule of law, and the security of our nations and the wider Indian Ocean region”.^[19] The preamble also noted that both parties have a “regard to the decisions of international courts and tribunals, including the International Court of Justice, relating to the Chagos Archipelago”, recognise “the wrongs of the past” and are “mindful of the need to complete the process of decolonisation of Mauritius”.

Article 1 states that Mauritius is sovereign over the Chagos Archipelago “in its entirety”, including Diego Garcia.^[20] Article 19 defines the archipelago as comprising the islands (including Diego Garcia), maritime zones (including the internal waters, territorial sea, archipelagic waters and the exclusive economic zone), airspace, seabed and subsoil (the continental shelf).^[21]

Article 2 outlines that, as the sovereign state, Mauritius authorises the UK to use Diego Garcia under the terms of the agreement.^[22] The authorisation includes all rights and authorities that the UK requires “for the long-term, secure and effective operation of the base”, covering defence and security requirements outlined in annex 1 and jurisdictional arrangements in annex 2 (detailed below). However, article 2(3) states that Mauritius retains sovereignty over the land and territorial sea of Diego Garcia and retains rights outside the scope of the agreement, such as management of natural resources, including fisheries and environmental conservation.

Article 3 states that both parties have agreed on defence and security measures outlined in annex 1.^[23] In accordance with the terms of the agreement, the UK shall respect Mauritian sovereignty over the Chagos Archipelago. Despite this, it sets out that the UK shall have “full responsibility” for the defence and security of Diego Garcia. The agreement also says that neither party shall “undermine, prejudice or otherwise interfere with the long-term, secure and effective operation of the base” and shall “cooperate to that end”. Additionally, the article states that both parties shall cooperate on matters relating to maritime security, including trafficking in arms, narcotics and people.

Article 4 sets out that both parties agree that activities related to the military base will comply with international law.^[24]

Annex 1 outlines the technical agreement regarding defence and security of the Chagos Archipelago, including Diego Garcia.^[25] Paragraph 1 states that Mauritius has agreed to unrestricted access, basing and overflight for UK and US aircraft and vessels entering the sea and airspace of Diego Garcia.^[26] Additionally, both parties agree the UK shall have “unrestricted ability” to conduct specific activities relating to the operation of the base, including:

• control over the conduct and deployment of armed operations and lethal capabilities
• deployment of and jurisdiction over personnel stationed in Diego Garcia
• control of access of individuals, goods, aircraft and vessels to Diego Garcia
• management, use and development of land and surrounding waters for defence purposes, excluding the construction of artificial islands

Annex 1(3) includes provisions regarding UK operations beyond Diego Garcia.^[27] Mauritius has agreed to:

• Unrestricted rights for UK and US vessels, aircraft, and allied states for overflight, navigation, and undersea access, with Mauritius notified if such access involves a state other than the UK or US.
• UK maintenance and upgrades of equipment after notifying Mauritius and advising on equipment locations.
• Joint UK-Mauritius decisions on managing and using the electromagnetic spectrum to safeguard the base’s secure operation.
• Authorisation requirements for non-UK, non-US, and non-Mauritian security forces, except in emergencies or natural disasters. Any presence beyond 30 days remains subject to joint approval.
• Establishing a maritime zone between 12 and 24 nautical miles around Diego Garcia, with joint decisions on constructing installations, sensors, structures, or artificial islands.

The annex also contains provisions for a security review to be undertaken by Mauritius in certain circumstances.^[28] Before approving any construction or positioning of maritime installations, artificial islands, sensors and structures beyond 24 nautical miles, or development within the Chagos Archipelago beyond Diego Garcia, Mauritius must conduct a security review under paragraph 6. The UK may present concerns if it believes a proposal “risks undermining, prejudicing or otherwise interfering with the long-term, secure and effective operation of the base”. Depending on UK-provided security assessments, Mauritius may agree not to proceed, request more information or refer the matter to the joint commission (see section 2.7 of this briefing), with the proposal proceeding only if it is jointly decided.

Described by the explanatory memorandum as a “further safeguard”, paragraph 7 provides that if the UK identifies an activity in the archipelago that is not subject to a security review but potentially conflicts with the agreement’s commitments to maintain the base’s secure and effective operation, the UK may issue a formal notification.^[29] Mauritius is then required to conduct a security review, following the procedures outlined in paragraph 6.

Annex 2 says the UK shall have the “prescriptive, enforcement and adjudicative criminal and civil jurisdiction necessary” to administer and operate the military base and related activities on Diego Garcia.^[30] Additionally, the UK may enter into agreements or arrangements with other states regarding jurisdiction over foreign civilian and military personnel stationed at the base (paragraph 2).

The annex also sets out that Mauritius shall exercise all “prescriptive, enforcement and adjudicative criminal jurisdiction” over Mauritian nationals who are not connected to the base and not members of the UK military or US personnel, and anyone else not connected to the operation of the base, including people involved in offences relating to unlicensed commercial fishing and arms or drugs trafficking.^[31] In cases of concurrent UK/Mauritian criminal jurisdiction, Mauritius has primary jurisdiction, although it may elect not to exercise it and may also exercise its jurisdiction where the UK chooses not to do so (paragraphs 4 to 6). Additionally, both parties commit to cooperate on the exercise of criminal jurisdiction (paragraphs 8 to 10).

2.2 Environment

Under article 5, the UK will exercise its rights and responsibilities at Diego Garcia in accordance with applicable international law on environmental protection and with “due regard to applicable Mauritian environmental laws”.^[32] The article also says the UK has agreed to support Mauritius in establishing and managing a marine protected area within the archipelago. The terms for this arrangement will be set out in a separate written instrument. Additionally, both parties agree to cooperate on other matters relating to the protection of the environment, including oil and other spills, and illegal, unreported and unregulated fishing.

2.3  Chagossians

Article 6 states that Mauritius is “free” to implement a resettlement programme on the islands of the archipelago, other than Diego Garcia.^[33] Additionally, article 11 states that the UK will capitalise a trust fund, to be established by Mauritius, for the benefit of Chagossians.

2.4 International obligations, arrangements and organisations

Article 7 states that both parties confirm that none of their existing international obligations or arrangements with a third-party will conflict with the agreement.^[34] It also states that neither party will enter future international arrangements that would conflict with the provisions of the UK-Mauritian agreement.

Article 8 says the UK will ensure its membership of international organisations is consistent with Mauritius’ sovereignty over the archipelago (article 1).^[35] Both parties agree to address arrangements with international organisations through a separate written instrument.

Under article 9, the UK will exercise jurisdiction and control over Diego Garcia, in accordance with the framework set out in annex 2.^[36]

2.5 Mauritian employees and contractors

Article 10 outlines the future arrangement regarding Mauritian employment and contracting opportunities linked to the military base at Diego Garcia.^[37] It notes that, while the UK may select civilian employees, contractors, and sources of supplies and equipment at its discretion, it should:

• employ suitably qualified Mauritian nationals on the base to the maximum extent practicable, consistent with relevant UK policies, laws, and regulations
• give preference to suitably qualified Mauritian companies for contracts related to services, equipment, materials, and supplies, again to the maximum extent practicable under applicable procedures

In cases where a qualified Mauritian company is not awarded a contract, the article states that, upon request, the UK will provide an explanation to the company concerned.

2.6  Costs

Article 11 and the finance exchange of letters sent by the UK government to the Mauritian government (attached to the agreement document) outline the UK’s financial commitments to Mauritius.^[38] The UK agrees to:

• Provide annual payments to Mauritius under the following terms:
  • For the first three years of the agreement, the UK will pay £165mn per year.
  • For the subsequent 10 years (years four to 13), the UK will pay £120mn per year.
  • For years 14 to 99, the UK will pay £120mn per year. This amount will increase in line with inflation.
• Capitalise a trust fund established by Mauritius for the benefit of Chagossians. This will be a single payment of £45mn, made on the first day of the month following the first anniversary of the agreement’s entry into, or within 21 days thereafter (unless an extension is agreed).
• Provide an annual grant of £45mn as part of a development framework for projects that “promote the ongoing economic development and welfare of Mauritius and its people”. This funding will be allocated for a period of 25 years, with payment commencing on the first day of the month following the third anniversary of the agreement’s entry into force, or within 21 days thereafter (again, unless an extension is agreed).

In the explanatory memorandum to the agreement, the UK government estimates the average annual cost during the initial 99-year period of the treaty to be £101mn in 2025/26 prices.^[39] The total projected cost of the finance package using a net present value methodology is £3.4bn.^[40] However, some have argued the costs will be much higher.^[41]

2.7 Governance, disputes, termination and entry into force

Article 12 and annex 3 outline the governance framework for implementing the treaty, including the establishment of a joint commission comprised of representatives from both parties.^[42] The joint commission will include one senior representative and four additional representatives from each party. Additionally, the US will have the right to propose items for discussion in the joint commission and to designate a representative to attend meetings to provide input and advice. The commission will meet at least twice a year, and more frequently if requested by either party. It will serve as the forum for discussion on treaty implementation, security, protection and activities within the archipelago. Decisions made by the joint commission will require agreement from both parties.

Article 13 outlines the duration of the agreement.^[43] It notes that the agreement shall have an initial duration of 99 years from the entry into force of the agreement. The agreement may be extended for an additional 40 years, provided both parties reach an agreement on the conditions for the extension no later than two years before the expiration of the initial agreement.

Article 14 sets out procedures for resolving disputes on the application or interpretation of the treaty.^[44] Disputes will first be submitted to the joint commission, and then escalated to consultations between the parties where necessary. In cases concerning “essential security interests” resulting from the treaty’s implementation, the parties may request direct consultations between their prime ministers.

Article 15 makes provision for disputes relating to the treaty’s termination.^[45] Mauritius may terminate the treaty only under two conditions, both of which involve UK actions:

• Failure by the UK to make a payment required under article 11 of the treaty.
• A serious threat by the UK or from the military base in Diego Garcia to Mauritius’ national interests. This includes an armed attack or the threat of an armed attack by the UK on Mauritius, or one originating directly from Diego Garcia.

Article 15(2) outlines the procedure if Mauritius invokes termination.^[46] If the UK does not accept Mauritius’ justification for terminating the treaty, the parties must meet in the joint commission “with a view to finding a mutually acceptable solution”. If no resolution is reached within three months from the first joint commission meeting, unless extended by agreement, the UK may commence arbitration to determine whether grounds for termination exist. If the UK does not commence arbitration, the treaty will be terminated.

Article 18 specifies that the treaty will enter into force on the first day of the first month following the receipt of the last notification from each party confirming the completion of their internal requirements and procedures.^[47] The explanatory memorandum notes that “Mauritian procedures will depend on Mauritian ministerial decisions, but will likely take no more than six months”.^[48] Additionally, article 13 states that the UK retains a right of first refusal regarding the use of Diego Garcia for 40 years beyond the initial 99-year period on the same terms offered to any third party.

The explanatory memorandum further states that the government will introduce legislation to implement the treaty “as soon as parliamentary time allows necessary”.^[49] This will include amendments to the British Nationality Act 1981, reflecting the fact the BIOT will no longer remain a British Overseas Territory. The legislation will also provide for the winding-up of the BIOT’s existing governance structures and establish powers to make secondary legislation for the “long-term, secure and effective operation of the base”. The treaty will only come into force when the “necessary primary and secondary legislation is in place”.

3. Political reaction

3.1 Conservative Party

Leader of the Opposition Kemi Badenoch has criticised the agreement.^[50] In an article on her website, published on 24 May 2025, Ms Badenoch described the agreement as a “surrender” and argued that the UK was “handing over British territory and paying upwards of £30bn to do so” to Mauritius, a country she claimed “openly cuddles up to China and Russia”. She continued:

Keir Starmer is using international law as a fig leaf for an ideological, political choice. There was no binding judgment against our sovereignty over the Chagos Islands, and no court has the jurisdiction to make one. The prime minister’s claim that we had to do this deal or face losing the base is a risible argument that doesn’t stand up to scrutiny. The text of this surrender treaty show[s] what this is really about: decolonisation and “righting the wrongs of the past”—snivelling capitulations to left-wing activists who hate Britain and are ashamed of our history.^[51]

In response to the defence secretary’s statement on the agreement in the House of Commons, the shadow defence secretary, James Cartlidge, also criticised the agreement.^[52] He stated:

The government are not surrendering British sovereign territory because of military defeat, or because of a binding legal verdict, but wilfully due to a total failure to take a stand and fight for Britain’s interests on the world stage—a complete and utter negotiating failure.

Additionally, Mr Cartlidge explained that his party had not signed an agreement while in government because they “fundamentally oppose the idea of spending billions of pounds on a surrender tax to lease back land that we currently own freehold”.^[53] He argued that a “key issue” was that the government “fear[ed] a binding legal judgment”, stating that the government was “following the legal advice to act definitively to our detriment, entirely on the basis of hypothetical risk that has not yet materialised and that we could challenge”. He also criticised the government for not consulting the Chagossian community, stating that they had been “shamefully sidelined […] from start to finish”.

Responding, John Healey argued that there was “no viable alternative option [to] this deal”.^[54] He highlighted that the previous Conservative government had held 11 rounds of negotiations with the Mauritian government before the general election in July 2024. Addressing concerns about not consulting with Chagossians, Mr Healey stated that Stephen Doughty, a minister at the Foreign, Commonwealth and Development Office, had “met them regularly” and that the government had “worked to ensure that the agreement reflects the importance that Chagossians attach to the islands”. He also highlighted that the UK was financing a trust fund to support the Chagossian community.

Discussing the legal implications of not reaching an agreement, Mr Healey warned that it could face a “range of international legal challenges and rulings against us”.^[55] This included the International Tribunal for the Law of the Sea, which the defence secretary described as “the most proximate, and the most potentially serious”.

In June 2025, Lord Callanan, the shadow foreign minister in the House of Lords, tabled a motion arguing against the ratification of the treaty.^[56] Speaking to the Telegraph on 2 June 2025, Lord Callanan explained his decision to introduce the motion:

The government’s unnecessary political decision to sign away our sovereignty over the Chagos Islands will cost the British taxpayer £30bn and put our strategic defence interests at risk […] This Chagos sellout shows Labour’s true priorities. That is why Conservatives are leading the fight against this shameful surrender deal in the House of Lords.

The motion is due to be debated in the Lords on 30 June 2025 and reads:

Lord Callanan to move that this House resolves, in accordance with section 20 of the Constitutional Reform and Governance Act 2010 and in the light of concerns about the cost of the agreement, the absence of any legal requirement to conclude such an agreement, its impact on international security, the lack of meaningful consultation of the Chagossian people, and recognising the right of Chagossians to be registered as British Overseas Territory citizens under the Nationality and Borders Act 2022, that His Majesty’s Government should not ratify the agreement between the United Kingdom and the Republic of Mauritius concerning the Chagos Archipelago including Diego Garcia, laid before the House on 22 May.

However, under the Constitutional Reform and Governance Act 2010 (CRAG Act 2010), a minister can override the House of Lords through a statement in the House of Commons.^[57]

3.2 Liberal Democrats

The Liberal Democrats’ defence spokesperson, James MacCleary, stated that his party “support[s] the UK complying with international law”.^[58] However, the party was critical of the government for its process in reaching an agreement, describing it as “more than a little bit bumpy”. Speaking in response to John Healey’s statement on the agreement, Mr MacCleary requested further information on the proposed payment schedule, the priorities of the Chagossian community, the future use of the base by the US military, and the mechanisms for scrutinising the agreement.

In response, John Healey stated that the government had published the “full financial information” and that “proper scrutiny by this House begins today”.^[59] He also stated that when the bill is published on the treaty the member “will be able to scrutinise that, too”. Additionally, he highlighted that the government had agreed to provide £40mn to establish a trust fund for Chagossians. Discussing the involvement of the US, the defence secretary explained that although the government had “kept the Americans informed and consulted them”, the US did not possess a veto over the agreement.

3.3 Peter Lamb MP

Following the signing of the agreement, Peter Lamb (Labour MP for Crawley, a constituency with a large Chagossian population) published a press release on his website.^[60] Mr Lamb criticised the agreement for containing “none of the legal certainties required to guarantee Chagossians—particularly those resident in the UK—the ability to access the islands or for permanent habitation to be possible”. He argued the agreement “hands these questions over to Mauritius and leaves it to them to decide whether to enable the future the Chagossian people seek or to use the deal for their own ends”. He also stated that this approach meant the government “will no longer have any ability to truly make amends for the abuses committed 60 years ago, [and] never have the ability to wipe away the stain it has left on our nation”.

3.4 House of Lords committee scrutiny

The House of Lords International Agreements Committee published its report into the agreement on 25 June 2025.^[61] The committee described the base on Diego Garcia as a “vital UK national asset” and stated that it agreed with the government that securing its long-term operation was “critical”.^[62] It also said the US government’s support for the agreement was “significant”.

However, the committee argued the agreement was “not perfect” and expressed several concerns.^[63] These concerns included:

• the cost to the UK taxpayer was too high
• the agreement did not “guarantee” that Chagossians can return to the Chagos Islands in the future
• there were uncertainties around the future of the marine protected areas
• there was no guarantee of an extension to the agreement following the initial 99-year period
• that questions have been raised concerning the enforceability of the right of first refusal

Despite this, the committee warned that if the agreement was not ratified, and that if a future government attempted to continue “resisting international pressure” to transfer sovereignty of the Chagos Islands, then Mauritius was “likely to resume its campaign through international courts with a view to obtaining a legally binding judgment on sovereignty against the UK”.^[64]

The House of Lords International Relations and Defence Committee also ran an inquiry on the subject. On 26 June 2025, Lord de Mauley (Conservative), wrote to Foreign Secretary David Lammy outlining the committee’s findings, with recommendations focused on four areas:^[65]

• Future of defence and security arrangements on Diego Garcia: The committee warned that the renewal provisions of the agreement could be a “potential source of future vulnerability in the face of growing military ambitions by China in the Indian Ocean”.^[66] Therefore, it called on the Foreign, Commonwealth and Development Office and the Ministry of Defence to address the issue as part of their long-term strategic planning. The committee also recommended that the government use the agreement as an “opportunity to reassess the future potential of Diego Garcia” to ensure the base meets current and future operational needs, “aligned with 21st century defence priorities”.^[67]
• Rights and welfare of Chagossians: The committee expressed concerns about the agreement’s impact on the Chagossian people. It said the government “missed the opportunity to involve the community in a more meaningful way throughout the negotiations”.^[68] Therefore, it recommended that the government establish a consultation mechanism with the Chagossian community to monitor the agreement’s implementation. The committee also called on the government to provide a “detailed explanation of the specific operational and logistical reasons for excluding Chagossians from the island [Diego Garcia]” and to engage with Mauritius to ensure that money from the trust fund is “transparently disbursed and benefit[s] the community directly”.^[69]
• Environmental protection: The committee raised concerns that the transfer of sovereignty could “compromise long-standing conservation efforts”.^[70] It recommended that the government work with Mauritius to ensure that a proportion of the development grant provided is used to support marine conservation and enforcement, including the establishment of a “comprehensive” marine protected area.
• Costs: The committee sought clarification from the government on how the leasing costs for Diego Garcia were determined and how these could affect the allocations in the defence budget for European defence.^[71] It also suggested that the government consider US contributions for the base and periodically review payment arrangements governing the use of Diego Garcia by the US.

4. Reaction to the agreement from Chagossians

On 22 May 2025, Bernadette Dugasse and Bertrice Pompe—two Chagossians living in the UK—filed a high court application to prevent the government from signing the agreement.^[72] Mr Justice Goose subsequently issued a temporary injunction, halting the government from signing the agreement. However, during a further hearing in the High Court that day, the government’s lawyer, Sir James Eadie, warned that “damage has already been done” in delaying the agreement and that “there is jeopardy to international relations”. The injunction was lifted by Mr Justice Chamberlain. Following the hearing, Jenny Simon, a campaigner on behalf of Chagossians, said that the government was “not treating us like we are human beings”.

In Mauritius, Oliver Bancoult, leader of the Chagos Refugees Group, described the signing of the agreement as “a historic day for us”.^[73] Additionally, Hisham Oozeer, a barrister, called the agreement a “win-win” for the UK and Mauritius. He stated that “today we have on one side the sovereignty of Mauritius over the Chagos Islands being confirmed, and also the safety element, which was important for UK and USA, being preserved”.

5. Reaction from UN officials

On 10 June 2025, four UN officials published a joint statement criticising the treaty, arguing that it “fails to guarantee and protect the rights of the Chagossian people, including their right to return to Diego Garcia, effective remedy and reparations and their cultural rights”.^[74] They stated that by the UK maintaining a military presence on Diego Garcia, it prevented Chagossian people from returning to the island and “appears to be at variance with the Chagossians’ right to return, which also hinders their ability to exercise their cultural rights in accessing their ancestral lands from which they were expelled”. They therefore called for the ratification of the treaty to be suspended and for a new agreement to be negotiated.

6. Parliamentary scrutiny of treaties

Since 1924, a convention (known as the Ponsonby rule) has established that once a treaty is signed it must be laid before Parliament for 21 days before ratification.^[75] In 2010, this convention was placed on a statutory footing in part 2 of the CRAG Act 2010.

However, treaties can be ratified without requiring a debate or vote in either House of Parliament. The House of Commons has the power to delay ratification for 21 days—repeatedly, if desired—provided the government makes time for debates and votes to take place. The House of Lords may vote against a treaty’s ratification. However, the government can ratify the treaty by making a statement setting out why the treaty should still be ratified.

Having been laid before both Houses of Parliament on 22 May 2025, the treaty’s objection period ends on 3 July 2025.^[76] As mentioned above, the House of Lords is due to consider a motion arguing against the ratification of the treaty on 30 June 2025.

Further information on Parliament’s scrutiny of treaties can be found in the following House of Lords Library briefing:

• House of Lords Library, ‘Parliamentary scrutiny of treaties’, 16 May 2023

7. Read more

• House of Commons Library, ‘2025 treaty on the British Indian Ocean Territory/Chagos Archipelago’, 30 May 2025
• House of Commons Library, ‘British Indian Ocean Territory: 2024 UK and Mauritius agreement’, 31 October 2024

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This briefing was updated on 26 June 2025 to include material from the House of Lords International Agreements Committee, published on 25 June 2025, and the House of Lords International Relations and Defence Committee, published on 26 June 2025.

Cover image by Earth Science and Remote Sensing Unit, NASA Johnson Space Center, on Wikimedia Commons.