The United Nations has ruled that Britain must give up control of the Chagos Islands in the Indian Ocean as "rapidly as possible" after decades of debate with Mauritius over the archipelago, which is also home to a huge US air-base.
"The United Kingdom is under an obligation to bring an end to its administration of the Chagos Archipelago as rapidly as possible, thereby allowing Mauritius to complete the decolonisation of its territory."
The UN General Assembly in 2017 adopted a resolution presented by Mauritius and backed by African countries asking the ICJ to offer legal advice on the island's fate and the legality of the deportations.
In 1965, Britain split off the islands from Mauritius -- which lies around 2,000 kilometres (1,200 miles) away -- three years before Port Louis gained independence. It also paid Mauritius three million pounds.
Between 1968 and 1973 around 2,000 Chagos islanders were evicted to Britain, Mauritius and the Seychelles in order to make way for a military base on Diego Garcia, the largest of the islands. The evictions were described in a British diplomatic cable at the time as the removal of "some few Tarzans and Man Fridays".
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.
"Our territorial integrity will now be made complete, and when that occurs, the Chagossians and their descendants will finally be able to return home," he said in a statement.
The ICJ opinion comes as a stunning blow to London in a case that goes to the heart of historic issues of decolonisation and current questions about Britain's place in the world as it prepares to leave the European Union.
Mauritius' lawyer Philippe Sands said there was "no wiggle room" in the judges' view and that Britain would resist pressure to comply.
"I suspect the United Kingdom will say to itself, what resistance can we put up to moving forward -- and particularly in the context of Brexit, as the United Kingdom finds itself a little bit isolated in the world," he told reporters outside court.
Britain's foreign ministry rejected the court's opinion.
"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 and Commonwealth Office said in a statement.