The United Nations General Assembly dealt Britain an embarrassing defeat on Wednesday in a protracted dispute over the Chagos Islands in the Indian Ocean, demanding that the British surrender the archipelago — home to an important American military base — to Mauritius, a former British colony.
In a diplomatic showdown that underscored challenges to Britain’s influence in parts of the world where, in a bygone era, it had commanded enormous power and territorial reach, the General Assembly voted 116 to 6 for the resolution, which said the British should withdraw their “colonial administration” from the Chagos Islands within six months.
The United States was among the opponents of the resolution, which is nonbinding and cannot be enforced. But it carries important weight as a barometer of world opinion.
Britain’s ambassador to the world body, Karen Pierce, who had implored General Assembly members to defeat the resolution, said her country had no plan to abandon the islands but that it was “disappointed by the results.”
The resolution, co-sponsored by several African nations, was put to a vote nearly three months after the highest court in the United Nations system had urged Britain to end its control of the islands “as rapidly as possible.”
That decision, an advisory opinion by the judges of the International Court of Justice in The Hague, was welcomed by officials from Mauritius, who saw it as an affirmation of their claim that Britain, as a colonial power, had wrongfully detached the Chagos Islands from Mauritius in 1965 before granting the colony independence.
“The court has characterized this as an unlawful act,” said Prime Minister Pravind Jugnauth of Mauritius, who spoke to the General Assembly before the vote. “One would have hoped that any country found to be engaged in an unlawful act by the highest court in the world would have hastened to make amends.”
He said that rejection of the resolution “would be nothing less than an endorsement of colonialism.”
The British, which renamed the islands the British Indian Ocean Territory, have always contended that the International Court of Justice should have no say in the matter, which Britain considered a sovereignty dispute with Mauritius that had nothing to do with colonial history.
The dispute is partly tied to the pre-1965 collaboration by Britain and the United States to construct a military base on one of the largest of the Chagos Islands, Diego Garcia. Once construction began, Britain evicted all Diego Garcians not connected to the base. None of the estimated 1,800 islanders, many now in their 70s or 80s, have been permitted to return.
The Diego Garcia base, which Britain has leased to the United States until 2036, has played an important role in American military operations, including bombing runs in Afghanistan and Iraq.
The Central Intelligence Agency has denied reports that the base was an interrogation site for suspected militants captured after the Sept. 11, 2001 attacks, but the agency has acknowledged that the base was twice used as a refueling stop for planes carrying detainees in 2002.
Ms. Pierce told the General Assembly before the vote that the base had played “a vital role in keeping allies and friends — including Mauritius — in the region and beyond safe and secure.”
Besides Britain and the United States, the other countries that opposed the resolution included Israel, Hungary, Australia and the Maldives. Fifty-six countries abstained, including Canada and Germany, and 15 countries did not vote.
It is unclear what — if anything — would happen to the Diego Garcia base if the British were to ever cede sovereignty to Mauritius. American officials have said British sovereignty is essential to the effective functioning of the base.
In his remarks, Mr. Jugnauth said Mauritius was prepared to enter into an agreement with the United States or Britain on the facility, saying it would “provide a higher degree of legal certainty” than what presently exists.