The British retained possession of the islands after Mauritius gained independence in 1968. The UK government calls them the British Indian Ocean Territory, or BIOT.

In 1971, about 1,500 natives of the islands were deported so that the largest island could be leased to the US for an airbase.

The islanders have never been allowed to return to their home.

President of the ICJ, Abdulqawi Ahmed Yusuf, said the detachment of the Chagos archipelago from Mauritius in 1965 was not based on a “free and genuine expression of the people concerned”.

“This continued administration constitutes a wrongful act,” he added.

“The UK has an obligation to bring to an end its administration of the Chagos archipelago as rapidly as possible”.

He added that the process of separating the Chagos Islands from Mauritius in the 1960s was “unlawful”.

In a submission to the court last year, Mauritius argued it was coerced into giving up the islands and that the separation breached a UN resolution passed in 1960 banning the breakup of colonies before independence.

However, the UK government argued the court lacked jurisdiction to hear the case, which will now be referred back to the UN General Assembly.

By a majority of 13 to one, the court held that the decolonisation of Mauritius had not been lawfully completed.

A single judge dissented, an American.

A Foreign Office spokesperson said: “This is an advisory opinion, not a judgment. Of course, we will look at the detail of it carefully. 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.”

Professor Philippe Sands QC, who represented Mauritius at The Hague, said: “The court has given a crystal-clear verdict, which upholds the rule of law. This is a historic and landmark judgment. It will be for Mauritius and the UK to sit down and implement this advisory opinion.

“It will be for Mauritius now to decide on the resettlement of the islanders. There’s no veto at the UN general assembly. It will decide how to go forward with the matter. There’s no question of the UK coming up with new arguments: their arguments were put forcibly and well.

“It’s difficult to imagine the UK as it moves forward into this Brexit world, ignoring what the International Court of Justice and the UN General Assembly have said.

“The UK is a country which prides itself on respect for the rule of law. Our hope and expectation is that the UK will honour the ICJ’s findings and give effect to it as rapidly as possible.”

Mauritius’s Prime Minister, Pravind Kumar Jugnauth, said: “This is a historic moment for Mauritius and all its people, including the Chagossians who were unconscionably removed from their homeland and prevented from returning for the last half-century.

“Our territorial integrity will now be made complete, and when that occurs, the Chagossians and their descendants will finally be able to return home.”