Britain has agreed to hand sovereignty of the long-contested Chagos Islands, an archipelago of more than 60 islands in the Indian Ocean, to Mauritius, in a deal Foreign Secretary David Lammy claims will secure the future of a strategically important UK-US military base at Diego Garcia.

But the UK Government’s decision has prompted a scathing reaction from political rivals, with former Home Secretary and Tory leadership candidate James Cleverly erupting at the “weak” decision.

The UK Government said it has reached a political agreement with Mauritius over the Chagos Islands, also known as the British Indian Ocean Territory, following negotiations that began in 2022.

Mauritius will assume sovereignty over the Indian Ocean archipelago while the joint US-UK military base remains on Diego Garcia, the largest of the islands.

The Foreign Office said the agreement is supported by the US and means the status of the base will be undisputed and legally secure.

The islands, though closer to India, are culturally associated with Africa, and Mauritius is part of the African continent geographically.

Chagossians were forced to leave by 1973 to make way for the military base.

The expulsions are regarded as one of the most shameful parts of Britain’s modern colonial history and Chagossians have spent decades fighting to return to the islands.

The United Nations’ highest court, the International Court of Justice, previously ruled the UK’s administration of the territory was “unlawful” and must end.

Mr Lammy said: “This Government inherited a situation where the long-term, secure operation of the Diego Garcia military base was under threat, with contested sovereignty and ongoing legal challenges.

“Today’s agreement secures this vital military base for the future.

“It will strengthen our role in safeguarding global security, shut down any possibility of the Indian Ocean being used as a dangerous illegal migration route to the UK, as well as guaranteeing our long-term relationship with Mauritius, a close Commonwealth partner.”

The Foreign Office said the agreement is subject to a treaty and supporting legal instruments being finalised, adding that both sides have committed to complete this as quickly as possible.

A Downing Street spokesman said Prime Minister Sir Keir Starmer spoke to his Mauritius counterpart, Pravind Jugnauth, on Thursday morning.

The spokesman said: “The leaders began by welcoming the political agreement achieved today between the UK and Mauritius on the exercise of sovereignty over the Chagos archipelago after two years of negotiations.

“The Prime Minister reiterated the importance of reaching this deal to protect the continued operation of the UK/US military base on Diego Garcia.

“He underscored his steadfast duty to national and global security which underpinned the political agreement reached today.”

For an initial period of 99 years, the United Kingdom will be authorised to exercise with respect to Diego Garcia “the sovereign rights and authorities of Mauritius required to ensure the continued operation of the base well into the next century”, a joint statement issued by the UK and Mauritian governments explained.

Mr Cleverly, said: “Weak, weak, weak! Labour lied to get into office. Said they’d be whiter than white, said they wouldn’t put up taxes, said they’d stand up to the EU, said that they be patriotic. All lies!”

Fellow Tory leadership candidate Tom Tugendhat said: “This is a shameful retreat undermining our security and leaving our allies exposed.”

He said the Foreign Office had “negotiated against Britain’s interest” and it was “disgraceful that these negotiations started under our watch”.

Mr Tugendhat added: “Lord Cameron rightly blocked them only to see it back under David Lammy’s complete failure of leadership.”

Robert Jenrick said: “It’s taken three months for Sir Keir Starmer to surrender Britain’s strategic interests.

“This is a dangerous capitulation that will hand our territory to an ally of Beijing.”

US President Joe Biden welcomed the news, saying the agreement “secures the effective operation” of the Diego Garcia military base.

He said: “I applaud the historic agreement and conclusion of the negotiations between the Republic of Mauritius and the United Kingdom on the status of the Chagos Archipelago.

“It is a clear demonstration that through diplomacy and partnership, countries can overcome long-standing historical challenges to reach peaceful and mutually beneficial outcomes.”

He added: “Diego Garcia is the site of a joint US-UK military facility that plays a vital role in national, regional, and global security.

“It enables the United States to support operations that demonstrate our shared commitment to regional stability, provide rapid response to crises, and counter some of the most challenging security threats we face.

“The agreement secures the effective operation of the joint facility on Diego Garcia into the next century.”