The Defence Secretary is understood to be alarmed by Foreign Office plans to negotiate a “Cyprus-style” deal to hand Mauritius the Chagos Islands, which have been British-owned since 1814.

The United Nations has called for the islands to be returned, with a UN court ruling Britain’s possession of the overseas territory “unlawful”.

Located in the Indian Ocean, 310 miles south of The Maldives, the Chagos Islands are home to the Diego Garcia military base, which is leased by the UK to the United States and has been used for American bombing missions in the Middle East. The archipelago is known in the UK as the British Indian Ocean Territory.

Lord Cameron, the Foreign Secretary, was approached by Mr Shapps and Oliver Dowden, the Deputy Prime Minister, after taking office last month and urged to scrap plans to give the islands away.

All three ministers are understood to be concerned that ceding them to Mauritius, which is allied with China, would endanger the UK’s “special relationship” with the US and its defence interests in the Indo-Pacific region.

The islands have been the subject of a decades-long dispute with Mauritius, which won a ruling at the International Court of Justice in 2019 to have Britain’s ownership declared unlawful and for the islands to be returned “as rapidly as possible”.

The court’s decision was welcomed by the United Nations general assembly, which voted by a large majority for the islands to be relinquished. The US and Australia voted with the UK for them to be retained by the Crown.

In 2021, a separate UN maritime court in Hamburg found that the UK’s control of the islands amounted to an “unlawful occupation”.

However, Mr Shapps, who took office in August, is understood to have told officials that the islands should remain under British control, rejecting the international court ruling and defying the UN.

“Grant is very against giving the islands back to Mauritius,” said a government source. “He and [Lord] Cameron came in, and said: ‘What the hell is going on here?’”

Their concerns are understood to be mirrored by the US, which believes the well-placed base could be compromised if Mauritius gained control of the islands.

Mauritius’s ally China is locked in competition with US forces in the Indo-Pacific region.

The US used Diego Garcia to fly bombing missions to Afghanistan in the aftermath of the 9/11 attacks, and to Iraq from 2003. Since 2006, it has been used as an “unsinkable aircraft carrier” facility to supply troops working in the Middle East.

The move to retain the islands would put the UK Government in conflict with international law and inflame tensions with the UN, but would protect Britain’s relationship with the US, sources suggested.

Boris Johnson, the former prime minister, said in September that it would be a “colossal mistake” to give away the Chagos Islands, describing a government plan to “haul down the flag, casting doubt on a major Western strategic asset”.

The Chagos Islands were originally acquired by the UK in 1814 under the Treaty of Paris, which saw Britain take control of Mauritius and its dependencies from France.

They were first split away from Mauritius in 1965, in the run-up to the island nation’s independence from the UK in 1968.

The UK expelled up to 2,000 indigenous people from the islands to Mauritius and the Seychelles to make way for the Diego Garcia base, prompting a human rights outcry that led to the establishment of a £40 million compensation fund for displaced locals in 2016.

A Foreign Office spokesman said: “Our clear aim is the continued full operation of the joint UK-US military base on Diego Garcia, which plays a crucial role in national, regional and global security.

“The UK will only enter into an agreement that protects our national interests and those of our partners.”