The Mauritian flag has been raised for the first time on the Chagos Islands, an Indian Ocean archipelago controlled by Britain but claimed by Mauritius.
Mauritian PM Pravind Jugnauth described it as an "historic moment", saying it was time for the UK to cede control.
The event was the culmination of a trip to the islands organised by the Mauritian government.
The Foreign Office said the UK had "no doubt" as to its "sovereignty over the British Indian Ocean Territory".
The ceremony, led by the Mauritian ambassador to the UN, took place on the atoll of Peros Banhos, where a pre-recorded message from Mr Jugnauth was played. A flag was also raised on Salomon, a separate atoll.
Three UN bodies have ruled that the archipelago - more than 5,000 miles away from the UK - is part of Britain's old empire and should be handed to Mauritius.