The UK should end its control of the Chagos Islands in the Indian Ocean "as rapidly as possible", the UN's highest court has said.
Mauritius claims it was forced to give up the islands - now a British overseas territory - in 1965 in exchange for independence, which it gained in 1968.
The International Court of Justice said the islands were not lawfully separated from the former colony of Mauritius.
The UK Foreign Office said: "This is an advisory opinion, not a judgment."
It added it would look "carefully" at the detail of the opinion, which is not legally binding.
The UK has previously said it will hand the islands back to Mauritius when they are no longer required for defence purposes.
Referencing that, the Foreign Office said: "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."