Britain said Thursday that it would cede sovereignty of the Chagos Islands to Mauritius in a deal it said secured the future of the UK-U.S. Diego Garcia military base, and which could also pave the way for people displaced decades ago to return home.

President Joe Biden welcomed the deal, saying it would secure the effective operation of Diego Garcia, a strategically important air base in the Indian Ocean, into the next century.

But critics in Britain said it was a capitulation that played into the hands of China, which has close trade ties with Mauritius. One group representing displaced Chagos Islanders expressed anger that they had been shut out of talks.

British Foreign Minister David Lammy said the deal settled the contested sovereignty of the islands, the last British overseas territory in Africa, while ongoing legal challenges had imperiled the long-term future of Diego Garcia.

He said the base, whose strategic significance was demonstrated during the Iraq and Afghanistan conflicts when it acted as a launch pad for long-range bombers, was now guaranteed for at least 99 years.

“Today’s agreement … will strengthen our role in safeguarding global security,” Lammy said in a statement.

Biden echoed that sentiment, saying Diego Garcia played “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,” he said.

Britain, 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, Britain evicted almost 2,000 residents to Mauritius and the Seychelles to make way for an air base on Diego Garcia, the largest island, which it had leased to the United States in 1966.

A nonbinding resolution in the United Nations General Assembly in 2019 said Britain should give up control of the archipelago after wrongfully forcing the population to leave.

In 2016, Britain’s Foreign Ministry extended Diego Garcia’s lease until 2036, and declared the expelled islanders would not be allowed to go back.

The new agreement said Mauritius would be free to implement a program of resettlement on the islands other than Diego Garcia, with the terms left for Port Louis to decide.

“We were guided by our conviction to complete the decolonization of our republic,” Mauritian Prime Minister Pravind Jugnauth said in a televised speech.

Olivier Bancoult, leader of the Mauritius-based Chagos Refugees Group, said it marked a decisive turning point and an official recognition of the injustices suffered by the Chagossians.

But British-based diaspora group Chagossian Voices said it deplored “the exclusion of the Chagossian community from the negotiations.”

“Chagossians … remain powerless and voiceless in determining our own future and the future of our homeland,” it said in a statement on Facebook.

British Prime Minister Keir Starmer, who has said his government would be defined in part by a respect for international law after his Labour Party won power in July, had made settling the issue a priority.

However, leading figures in Britain’s opposition Conservative Party, which initially launched the talks while in government, criticized the agreement.

Conservative security spokesman Tom Tugendhat said the deal undermined Britain’s allies and opened the possibility of China gaining a military foothold in the Indian Ocean.

“This is a dangerous capitulation that will hand our territory to an ally of Beijing,” Robert Jenrick, the favorite to be the next Conservative leader, said on X.

Asked about the concerns over China, State Department spokesperson Matthew Miller said: “The provisions of the agreement do give us assurances that we will be able to maintain the security of our base.”

Miller declined to detail the provisions in question.

David Blagden, associate professor of international security and strategy at Britain’s University of Exeter, said the deal was a “big win” for Mauritius.

“Not only will the UK pay Port Louis for ‘taking back’ an archipelago it’d never held sovereignty over, but they’ll now be able to extract lots of juicy Chinese aid in exchange for complicating US/UK use of Diego Garcia,” he said on X.