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Research & Analysis Published 30 Oct 2025 House of Lords Library ↗ View on Parliament

Diego Garcia Military Base and British Indian Ocean Territory Bill

Type: Lords Library Note (LLN-2025-0040) The Diego Garcia Military Base and British Indian Ocean Territory Bill would provide for the implementation of the UK-Mauritius agreement on the Chagos archipelago in UK domestic law ahead of the government ratifying the treaty. In particular it would end UK sovereignty over the archipelago and make certain consequential changes to nationality law, while also providing for the continued administration of the existing joint UK-US military base on the island of Diego Garcia.

Attachments

↓ LLN-2025-0040.pdf
application/pdf

▤ Verbatim text from source document

Diego Garcia Military Base and British Indian Ocean Territory Bill - House of Lords Library

Documents to download

Diego Garcia Military Base and British Indian Ocean Territory BillHL Bill 139 of 2024-26
(477 KB
, PDF)

Download full report

Download ‘Diego Garcia Military Base and British Indian Ocean Territory BillHL Bill 139 of 2024-26’ report (477 KB
, PDF)

Supporting documents

Large print briefingDiego Garcia Military Base and British Ocean Territory Bill

Large print briefingDiego Garcia Military Base and British Ocean Territory Bill
(493 KB
, PDF
)

(493 KB
, PDF)

The
Diego Garcia Military Base and British Indian Ocean Territory Bill
comprises six clauses and the government has published
explanatory notes
, a
delegated powers memorandum
and a
human rights memorandum
to accompany the bill.

The
British Indian Ocean Territory (BIOT)
is a British overseas territory administered from London. It comprises 640,000 square kilometres (sq km) of ocean around the islands of the Chagos archipelago located in the central Indian Ocean, approximately halfway between Tanzania and Indonesia. The island group’s largest island, Diego Garcia, which accounts for around half the archipelago’s total land area of 60 sq km, hosts a joint UK-US military facility.

On 22 May 2025 the
UK and Mauritius signed a bilateral treaty
concerning the future of the archipelago. The UK agreed to transfer sovereignty over the islands to Mauritius while retaining the right to exercise authority over Diego Garcia—allowing for the continued operation of the joint UK-US military base—for an initial 99-year period, with further extensions possible. The agreement included other provisions, including for an economic partnership under which the UK will make annual payments to Mauritius averaging £101mn in 2025/26 prices.

The
US government has publicly endorsed the treaty
on the basis that it “secures the long-term, stable, and effective operation of the joint US-UK military facility at Diego Garcia”.

However, the UK is a dualist state, meaning international treaties do not automatically change UK domestic law.
Foreign, Commonwealth and Development Office (FCDO) guidance on treaties
explains the government therefore considers it “essential” the UK “does not become legally bound until it has in place the necessary domestic powers to give effect to the terms of [a] treaty; otherwise it will be in breach of its international obligations”. The government may therefore ask Parliament to consider legislation required to implement an agreement’s provisions in UK domestic law.

The bill would provide for the changes required to domestic law before the government ratifies the treaty. The bill therefore constitutes part of the UK’s domestic procedures to implement the treaty.

The House of Commons has passed the bill. The House of Lords is scheduled to consider the bill at second reading on 4 November 2025.

Related Links
Link to bill page on UK Parliament website