The UN had given the U.K. six months to hand control of the Chagos Islands — an archipelago in the central Indian Ocean — back to Mauritius, but this deadline has now passed.
The government of Mauritius has previously accused Britain of being an "illegal colonial occupier" in the Chagos Archipelago.
The island state claims it was forced to hand over the Chagos Islands to the U.K. in 1965 for independence, which it gained three years later.
"The U.K. has no doubt as to our sovereignty over the British Indian Ocean Territory (BIOT), which has been under continuous British sovereignty since 1814," Britain's Foreign and Commonwealth Office (FCO) said in a statement earlier this month.
"Mauritius has never held sovereignty over the BIOT and the U.K. does not recognise its claim."
Britain's FCO was not immediately available for comment when contacted by CNBC on Friday morning. It has previously insisted it has every right to hold onto the islands.
On May 22, the UN General Assembly voted by an overwhelming majority of 116 to six to demand the U.K. "unconditionally end its occupation of the Archipelago" within six months. There were 56 abstentions.
The UN said the decolonization of Mauritius "was not conducted in a manner consistent with the right to self-determination" and "continued administration of the Archipelago constitutes a wrongful act."
The move came only three months after the UN's high court advised Britain to leave the Chagos Islands as soon as possible.
The deadline was not binding, and the U.K. is not expected to face sanctions or immediate punishment.
Alongside the U.K. and four others, the U.S. was one of the nations to vote against the UN resolution earlier this year. The world's largest economy has a military airbase on Diego Garcia, one of the islands that make up the Chagos Archipelago.
The Chagos Islands were separated from Mauritius in 1965 when the island state was still a British colony.
The U.K. reportedly purchased the archipelago for $3 million — thus creating the BIOT.
Located off the southeast coast of Africa, Mauritius has a population of approximately 1.3 million.
The country's economy has made "great strides" since gaining independence in 1968, according to the World Bank, but transitioning to a knowledge-based economy and adapting to the impacts of climate change remain key challenges.