UNITED NATIONS — Britain suffered a diplomatic blow on Thursday as the United Nations General Assembly voted to refer London’s territorial claim to one of its last colonial outposts to an international tribunal for its opinion on the dispute.

Notably, in the wake of Britain’s decision to exit the European Union, many countries of the bloc did not side with Britain. The United States did.

At issue is whether Britain has a claim to a strategically important Indian Ocean archipelago, known as the Chagos Islands. One of the islands, Diego Garcia, is the site of a British-American military base.

Mauritius, a former British colony that gained independence in 1968, says the islands are part of its territory and wants the International Court of Justice to offer its opinion. It casts the British claim as part of an unfinished struggle to free Mauritius from colonial rule.