The Diego Garcia Military Base and British Indian Ocean Territory Bill 2024-25 is a government bill (bill 285 of the 2024–25 parliamentary session). The bill was introduced on 15 July 2025 and received its second reading on 9 September 2025. The bill’s committee and other remaining Commons stages are scheduled for 20 October 2025.
The government has published explanatory notes, a delegated powers memorandum and a human rights memorandum, which are available on the ‘bills publications page.
The bill’s primary purpose is to implement in UK law certain provisions of the treaty signed in May 2025 between the UK and Mauritius agreeing the status and future of the Chagos Archipelago, including the joint US–UK military base on Diego Garcia (the archipelago’s largest island).
The UK currently administers the Chagos Archipelago as the British Indian Ocean Territory (BIOT), one of the UK’s 14 Overseas Territories.
Main provisions of the UK–UK-UK-Mauritius treaty
The treaty provides for Mauritius to exercise full sovereignty over the Chagos Archipelago, with the UK exercising rights on Diego Garcia during an initial 99-year period. Over the 99 years, the UK will pay Mauritius a total of around £3.4 billion in 2025/26 prices. The UK Government says this resolves the long-standing BIOT sovereignty dispute and secures the military base for continued operations in the long term.
Further details of the treaty are set out in Library briefing 2025 treaty on the British Indian Ocean Territory/Chagos Archipelago.
What does the bill do?
Under the UK’s constitution, the formal rights and duties for the UK in international law do not automatically change UK domestic law. Legislation is therefore sometimes needed to give domestic effect to new treaty-based rights and duties that the government has agreed to.
This bill contains the provisions of the treaty that require changes to UK law. These include:
- terminating the UK’s sovereignty over the BIOT but providing for the continued administration of Diego Garcia by the UK
- revoking UK legislation that provides the BIOT’s constitution and certain provisions relating to the citizenship status of those connected to the BIOT
- providing powers to implement the treaty using the Crown’s prerogative powers through Orders in Council
- saving existing laws applying to the BIOT and applying them to Diego Garcia, except for British nationality law
- removing the ability of people to acquire British Overseas Territories citizenship based on connection to the BIOT, while preserving a route to full British citizenship for Chagossian descendants
UK Government support for the treaty
Announcing the agreement on 22 May 2025, Prime Minister “”Sir Keir Starmer said the deal was absolutely vital for the “safety and security of the British people”. He said it was needed to end the ongoing UK–UK-UK-Mauritius sovereignty dispute and to guarantee the military base. The government has also pointed to the ‘USs support for the deal and argued that there is a risk of a future binding legal judgment affecting the UK’s ability to use the military base.
Reaction to the treaty and the bill
The –Conservative Party is opposed to the UKUK-UK-Mauritius treaty. It has been said that the costs to be paid to Mauritius are unacceptable, that the agreement puts the Diego Garcia base at risk, with Mauritius having ties to China, and has described the government’s concerns for potential legal judgments as “overblown”.
The Liberal Democrats say they support the UK acting in line with international law, but have called for more information on the payments and for reassurance that the US will only use the base in line with UK foreign policy interests and principles.
Some Chagossian campaign groups, such as Chagossian Voices, oppose the agreement and are in favour of the right of self-determination and the full right of return to the archipelago, including Diego Garcia. Bernadette Dugasse and Bertrice Pompe, both born on the Chagos Archipelago but now living in the UK, unsuccessfully sought to stop the agreement through an injunction in the High Court.1
In Mauritius, Olivier Bancoult, leader of the Chagos Refugees Group, said that “”the agreement represented a historic day for us and welcomed the opportunity for Chagossians to return to the archipelago.
Parliamentary scrutiny
In a June 2025 report, the –House of Lords International Agreements Committee examined the UKUK-UK-Mauritius treaty, saying the agreement was a “compromise between the views of the two Parties” and “it is not perfect”. It raised concerns over the costs to UK taxpayers, and the lack of guarantees that Chagossians can return to the outer islands, and that the initial 99-year lease can be extended. The committee argued, however, that the risks of not coming to an agreement were high and could affect the future of the military base, which it called a “vital UK national asset”.
On 10 June 2025, the Shadow Foreign Secretary, Priti Patel, introduced a presentation bill that includes provisions to prohibit the payment of public funds to another government in connection with the BIOT without parliamentary approval, and to require the government to consult with British Chagossians. Presentation bills are unlikely to become law.
On 30 June 2025, the House of Lords debated two motions against the ratification of the treaty.
The first, put down by the Conservative Lords Foreign Affairs spokesperson, Lord Callanan, declined to ratify the treaty. This was disagreed on the division.
The second motion, put forward by the Liberal Democrat Lords Foreign Affairs spokesperson, Lord Tweed, required further detail on the effects of the treaty on Chagossians. Lord Tweed withdrew the motion after the government committed to making a ministerial statement before ratification, “providing a factual update on eligibility for resettlement and the modalities of the trust fund”.
Progress of the bill
During the bill’s second reading debate, the Conservative, Liberal Democrat, and Reform parties expressed opposition to the legislation and the treaty. All put down reasoned amendments declining to give the bill a second reading.
Only the Conservative Party’s reasoned amendment was selected by the Speaker for decision. This was voted down, with 116 votes in favour and 333 against.
The motion to give the bill a second reading was approved by the House by 330 votes to 179.
SOURCE: commons library.parliament.uk
