The UK has been branded an "illegal colonial occupier" by Mauritius's prime minister for refusing to return the Chagos Islands by the United Nations-backed deadline.

The six-month schedule to return control of the overseas territory came and was set to pass on Friday, with the UK refusing to recognise Mauritius's claim of sovereignty over the islands.

Labour leader Jeremy Corbyn accused the Conservatives of "shamefully" considering themselves above international law.

Speaking in anticipation of the deadline's passing, Mauritius's prime minister Pravind Jugnauth said his government "strongly deplores" the UK's decision.

"This situation clearly leaves the United Kingdom as an illegal colonial occupier," he told Mauritius's national assembly.

The UN overwhelmingly voted in May to set the six-month deadline for UK withdrawal from the Indian Ocean archipelago, in a major diplomatic blow.

The Labour leader, speaking to reporters on the campaign trail in Stoke-on-Trent on Friday, said he would "absolutely" return the islands to Mauritius.

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"It's clear that in refusing to return the Chagos Islands to Mauritius and defying the UN General Assembly and International Court of Justice, this Conservative Government shamefully considers itself to be above international law," Corbyn added in a statement.

"We immediately will enact our manifesto promise to allow the people of the Chagos Islands and their descendants the right to return to the lands from which they should never have been removed."

The UK evicted Chagossians from the archipelago between 1967 and 1973 so that the US could erect a military base on Diego Garcia, the largest island in the group.

The National: The airbase on Diego GarciaThe airbase on Diego Garcia

That move, and the islands' incorporation into the British Indian Ocean Territory, was ruled "unlawful" by judges at the International Court of Justice in The Hague.

While not legally binding, the UN vote heaped diplomatic pressure on Britain to return the territory, with the General Assembly backing the resolution 116 votes to six.

In the speech on today, Jugnauth said: "The United Kingdom cannot profess to be a champion of the rule of law and human rights whilst maintaining an illegal colonial administration in part of the territory of Mauritius, and preventing the return to the Chagos archipelago of the former inhabitants it forcibly removed five decades ago, thereby being in clear violation of international law."

The UK purchased the archipelago from Mauritius for £3 million in 1965, when it was still a British colony.

Mauritius, which gained independence from Britain in 1968, maintains the islands are its own.

Asked for a comment, the Foreign Office pointed to a recent ministerial statement saying: "The UK has no doubt as to our sovereignty over the British Indian Ocean Territory (BIOT), which has been under continuous British sovereignty since 1814.

"Mauritius has never held sovereignty over the BIOT and the UK does not recognise its claim."