The ruling further complicates the UK’s government’s ongoing efforts to hand over control of the islands to Mauritius.
Britain has controlled the Chagos Islands since 1814 and evicted its inhabitants in the 1960s to make way for a military base on the largest island, Diego Garcia.
The islands became formally established as an overseas territory in 1965, after the British government paid Mauritius a £3m grant to retain the archipelago.
But last year, the UK government agreed to hand over control of the islands to Mauritius, arguing the future of the military base had been threatened by recent international court decisions.
Under the deal, the UK has agreed to pay £101m annually for 99 years to keep control of the UK-US military base.
The deal is now in doubt, after US President Donald Trump posted on his Trust Social platform to urge Prime Minister Sir Keir Starmer to not “give away Diego Garcia”.
The deal has also not gone down well with many people of Chagossian descent, who were granted the right to claim British citizenship in 2022.
Many Chagossians see the deal as a betrayal and want to see the UK retain sovereignty over the islands so they can one day return to their homeland.
But while many do oppose the deal, Chagossians – who live in the UK, Mauritius and the Seychelles – do not speak with one voice.