The United Kingdom has announced it is giving up sovereignty of the Chagos Islands to Mauritius in a deal that will secure the future of a key US military base on the atoll of Diego Garcia while allowing islanders displaced for the past five decades a right of return.

The deal, announced jointly by the UK and Mauritius on Thursday, grants the latter full sovereignty over the remote archipelago, guaranteeing the operation of the United States base for the next 99 years.

“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,” British Foreign Minister David Lammy said in a statement.

“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.”

The UK, which has controlled the region since 1814, detached the Chagos Islands in 1965 from Mauritius – a former colony that became independent three years later – to create the British Indian Ocean Territory.

In the early 1970s, it evicted about 1,500 residents to Mauritius and the Seychelles to make way for an airbase on the largest island, Diego Garcia, which it had leased to the US in 1966 in return for a $14m discount on Polaris missiles.

To avoid breaching international law, the UK had labelled the Chagossians, whose ancestral links to the territory go back to the late 18th century, as “transient workers”.

Former Mauritian Prime Minister Navin Ramgoolam launched a legal battle to win back the territory after a US cable was released by WikiLeaks in 2010, in which a Foreign Office official labelled Chagossians fighting for a right of return as “Man Fridays”.

Mauritius won back sovereignty in 2019, the International Court of Justice (ICJ) issuing an advisory opinion that the UK should give up control of the islands, saying it had wrongfully forced the population to leave in the 1970s to make way for the US airbase.

A subsequent UN resolution endorsed by 116 member states gave the UK six months to hand back the archipelago, but members of the previous Conservative government – including former Prime Minister Boris Johnson – had raised objections that Mauritius might allow China access to the territory, and negotiations stalled.

Critics said that a failure to abide by the terms of the ICJ ruling harmed the UK’s standing on the world stage, demonstrating a lack of respect for the rules-based international order.

The joint statement issued by the UK and Mauritius on Thursday stated that Mauritius “will now be free to implement a programme of resettlement on the islands of the Chagos Archipelago, other than Diego Garcia”.

The UK has pledged to set up a new trust fund and other support for Chagossians, who now number an estimated 10,000 people scattered across Mauritius, the Seychelles and the UK.

Some Chagossians, however, dispute Mauritian sovereignty and are campaigning for self-determination as an indigenous people.

Chagossian Voices, a community organisation based in the UK, had fought  to be included in the bilateral talks and slammed the UK for failing to consult them over the handover to Mauritius.

“Chagossian Voices deplore the exclusion of the Chagossian community from the negotiations which have produced this statement of intent concerning the sovereignty of our homeland,” said the group on Thursday in a statement on X.

“Chagossians learned this outcome from the media and remain powerless and voiceless in determining our own future and the future of our homeland. The views of Chagossians, the Indigenous inhabitants of the islands, have been consistently and deliberately ignored and we demand full inclusion in the drafting of the treaty,” it said.

As part of the deal, the UK will also provide Mauritius with a package of financial support to implement projects that boost the economic development of the country.

The two nations will also work together on issues including environmental protection, maritime security, drugs and people trafficking.

The UK and Mauritius said that the political agreement had the support and assistance of the US and India.

“The treaty will open a new chapter in our shared history,” it said, as it would “herald a new era of economic, security and environmental partnership between our two nations.”

US President Joe Biden on Thursday applauded the “historic agreement” between the UK and Mauritius.

“It is a clear demonstration that through diplomacy and partnership, countries can overcome longstanding historical challenges to reach peaceful and mutually beneficial outcomes,” Biden said in a White House statement.

Diego Garcia is nicknamed the “Footprint of Freedom” both because of its shape and its strategic location in the Indian Ocean, within striking distance of Africa, the Middle East and Southeast Asia.

It played a key role in the US overseas operations after the 9/11 attacks in 2001, sending aircraft to Iraq and Afghanistan. Controversially, rendition flights landed on the island in 2002, a charge long denied by the US, but confirmed by the UK government in 2008.