"The defence facilities on the British Indian Ocean Territory help to protect people here in Britain and around the world from terrorist threats, organised crime and piracy," the foreign ministry said in a statement.

"This is an advisory opinion, not a judgment," it said, adding however that Britain would "carefully" examine it.

The International Court of Justice said that Britain had illegally split the islands from Mauritius before its independence in 1968, after which the entire population of islanders was evicted.

Between 1968 and 1973 around 2,000 Chagos islanders were evicted, to Britain, Mauritius and the Seychelles, to make way for a military base on Diego Garcia, the largest of the islands.

Diego Garcia is now under lease to the United States and played a key strategic role in the Cold War before being used as a staging ground for US bombing campaigns against Afghanistan and Iraq in the 2000s.

Britain has said the islands will be returned to Mauritius when the need for military facilities ends.