The British government announced Thursday it will transfer sovereignty of the Chagos Islands to Mauritius while ensuring the continuation of the key U.K.-U.S. military base on the strategically important archipelago.

British Foreign Secretary David Lammy said the agreement secures the vital military base at Diego Garcia, the largest in the chain of islands, for the future.

The U.K. government said without the deal the secure operation of the military base would be under threat, with contested sovereignty and legal challenges, including through various international courts and tribunals. As part of the deal, the U.K. will retain the sovereignty of Diego Garcia for an initial period of 99 years.

"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 U.K., as well as guaranteeing our long-term relationship with Mauritius, a close Commonwealth partner," Lammy said.

The deal was strongly supported by international partners including the U.S., British officials said.

The agreement will have to be signed off in a treaty and depends on finalized legal processes. Both sides have committed to complete this as quickly as possible.

The Chagos Islands have been at the heart of what Britain calls the British Indian Ocean Territory since 1965 when they were siphoned away from Mauritius, a U.K. territory that gained independence three years later.

The U.S. Navy base at Diego Garcia was built in the 1970s and provides what American authorities have described as "an all but indispensable platform" for security operations in the Middle East, South Asia and East Africa.

Around 1,500 inhabitants from the Chagos Islands were displaced to make way for the U.S. base. It's unclear immediately whether they and their descendants, who are mainly living in the U.K., Mauritius and the Seychelles, will have a right to return.

In a statement, the White House said President Joe Biden applauded the "historic agreement" on the status of the Chagos Islands. It called the Diego Garcia base vital in preserving "national, regional, and global security."

"The agreement secures the effective operation of the joint facility on Diego Garcia into the next century," the statement said.

"This agreement affirms Mauritian sovereignty over the Chagos Archipelago while granting the United Kingdom the authority to exercise the sovereign rights of Mauritius with respect to Diego Garcia," it added.