2025 treaty on the British Indian Ocean Territory/Chagos Archipelago

On 22 May 2025, the governments of the UK and Mauritius signed an agreement on the sovereignty and future of the Chagos Archipelago, governed by the UK as the British Indian Ocean Territory (BIOT). The Archipelago includes the US–UK military base, Diego Garcia.

The BIOT is currently one of 14 UK Overseas Territories. The agreement provides for Mauritius to exercise full sovereignty over the Chagos Archipelago, with the UK exercising rights on Diego Garcia. The UK Government says this resolves the long-standing BIOT sovereignty dispute and secures the military base for continued operations in the long-term.

This briefing sets out the contents of the treaty, parliamentary reaction and scrutiny. A bill to implement parts of the treaty in UK law, the Diego Garcia Military Base and British Indian Ocean Territory Bill, was introduced on 15 July 2025. Second reading is scheduled for 9 September 2025. The Library bill briefing has more.

What has been the dispute over the BIOT?

The BIOT has been subject to long-running legal disputes, including:

• Sovereignty over the BIOT since it was created as a legal entity by the UK in 1965, detaching the Chagos Archipelago from Mauritius. From the 1980s onwards, Mauritius claimed sovereignty over the islands in international forums, and some international courts found (in non-binding decisions) that Mauritius, not the UK, held sovereignty over the BIOT. TheUK Government undertook that it would “cede sovereignty of the territory to Mauritius” once the BIOT was “no longer required for defence purposes”.
• The rights of the inhabitants of the Chagos Archipelago and their descendants to return to live on the islands after they were removed by the UK Government between 1968 and 1973, for the military base.

What does the 2025 agreement say?

The agreement has 19 articles, and is accompanied by an explanatory memorandum.

It says Mauritius is sovereign over all the Chagos Archipelago. The UK will have the right to exercise authority over the Diego Garcia military base, and Mauritius will allow the US and UK to access, maintain, and invest in the base for an initial 99-year period, and it will not allow other powers to use the outer islands around Diego Garcia without agreement with the UK. The period can be extended by a further 40 years, if both parties agree, and extended again after this. The UK will pay Mauritius an annual average of £101 million for 99 years in 2025/26 prices, totalling around £3.4 billion. The estimate has been verified by the government’s actuary department.

Mauritius will be free to arrange for resettlement of Chagossians on all the islands of the archipelago except Diego Garcia. It will also establish a marine protected area, with UK support, to protect the environment.

Mauritian Prime Minister, Navin Ramgoolam, said as a result of the agreement that “we have gained recognition of our sovereignty […] which completes the process of decolonisation which began in 1968”.

Why does the UK Government say the agreement is necessary?

Announcing the agreement on 22 May, 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–Mauritius sovereignty dispute and to guarantee the military base. The US supports the deal.

Mr Starmer cited three main reasons for the agreement:

1. to guarantee the joint US–UK military base for the long term
2. to avert the risk of a future binding legal judgment affecting the UK’s ability to use the military base (for example, the management of satellite communications from the base or access for contractors)
3. to secure the base and the surrounding archipelago against China or other states setting up a presence there

The government has said the costs represent less than 0.2% of the annual defence budget and are in line with wider practice for paying to maintain overseas military bases. There are no lease costs in current arrangements.

What has been reaction in the UK Parliament?

Conservative Shadow ministers have expressed opposition to the agreement. They have said the costs to be paid to Mauritius are unacceptable, the agreement puts the Diego Garcia base at risk, with Mauritius having ties to China, and have described the government’s concerns for potential legal judgments as “overblown”.

The Liberal Democrats back the UK acting in line with international law but have called for more information on the payments and for reassurance the US will only use the base in line with UK foreign policy interests and principles.

What parliamentary scrutiny has there been?

The agreement was laid before both Houses of Parliament on 22 May 2025.

Under part 2 of the Constitutional Reform and Governance Act 2010 (CRAG), both Houses had 21 sitting days (running to 3 July 2025) in which to pass a resolution to oppose the treaty’s ratification.

On 4 June 2025, two early day motions were tabled, both expressing opposition to ratifying the UK-Mauritius treaty.

The first early day motion, sponsored by the Leader of the Opposition, Kemi Badenoch, received 107 signatures.

However, early day motions carry no entitlement to a House of Commons debate or vote within the CRAG scrutiny period. No motion was passed to oppose ratification.

On 30 June 2025, the House of Lords debated two motions against the ratification of the treaty. The first, tabled by Shadow Minister for Foreign, Commonwealth and Development Affairs, Lord Callanan was not passed after being put to a vote. The second, tabled by Liberal Democrat Spokesperson for Foreign and Commonwealth Affairs, Lord Purvis of Tweed, was withdrawn after the government gave assurances it would publish further details on the effects of the treaty on Chagossians before ratifying the treaty.

On 3 July 2025, the 21-day sitting day period ended without a resolution to oppose the treaty’s ratification being passed. The government can now ratify the treaty. However, it is the long-standing practice of successive UK governments not to ratify a treaty unless and until it is able to implement that treaty in domestic law. The Diego Garcia Military Base and British Indian Ocean Territory Bill implementing the treaty has started its consideration in the House of Commons.

Mauritius will also need to complete its own internal processes to ratify the treaty.