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.