The UN’s highest court has said the UK should end its control of the Chagos Islands in the Indian Ocean.
The move should happen “as rapidly as possible”, according to the court.
The court ruled that Britain carved up Mauritius illegally when it agreed to end colonial rule and must "bring to an end its administration of the Chagos Archipelago".
The advisory opinion issued by the International Court of Justice is non-binding, but carries weight since it came from the top UN court and could put significant pressure on London to act.
Britain evicted about 2,000 people from the Chagos archipelago in the 1960s and 1970s so the US military could build an air base on Diego Garcia.
Many resettled in the UK and have fought in British courts to return to the islands.
The court said it concluded that even though Mauritius's government at the time of the country's 1968 independence agreed to split off the Chagos Islands, "this detachment was not based on the free and genuine expression of the will of the people concerned".
The islands are currently known as the British Indian Ocean Territory.
The court said it is now up to the United Nations General Assembly to decide how to "ensure the completion of the decolonisation of Mauritius".
While the United States still has a military base on Diego Garcia, the court said that in its opinion "all member states are under an obligation to co-operate with the United Nations in order to complete the decolonisation of Mauritius".
Additional reporting by Associated Press
