On Monday, Robert Jenrick said David Lammy had signed the deal so that he could “feel good about himself at his next north London dinner party”.

In a debate discussing the decision in Parliament, the Tory leadership contender said: “We’ve just handed sovereign British territory to a small island nation which is an ally of China – and we’re paying for the privilege.

“All so that the foreign secretary can feel good about himself at his next North London dinner party.”

‘Unsustainable’ legal position

However, the Foreign Secretary told MPs on Monday that the dispute between Britain and Mauritius was “clearly not sustainable” and that Labour faced a choice between “abandoning the base altogether or breaking international law”.

Friends of the British Overseas Territories, a charity dedicated to British-owned islands abroad, called Mr Lammy’s statement “shameful”.

“Proceeding with the transfer of [the island] goes against our national interests and must be stopped at once,” it said.

The ICJ had already issued a non-binding ruling that the islands belong to Mauritius, and a further ruling that forced the handover of the base was likely, he said, because of the “regrettable” removal of indigenous islanders by the UK in the 1960s.

Downing Street insisted the deal to give up sovereignty over the British Indian Ocean Territory (BIOT) was due to the “unsustainable” legal position and had no impact on other disputed territories including the Falkland Islands and Gibraltar.

The Prime Minister’s official spokesman would not be drawn on the cost to the UK taxpayer of the deal which will see Mauritius being given sovereignty over the islands, with a 99-year agreement to secure the strategically important UK-US military base on Diego Garcia.

The spokesman said: “The Government inherited a situation where the long-term secure operation of the military base at Diego Garcia was under threat with contested sovereignty and legal challenges, including through various international courts and tribunals.

“You will be aware that the previous government initiated sovereignty negotiations in 2022 and conducted a number of rounds of negotiations. This Government picked up those negotiations and has reached an agreement, which means that for the first time in over 50 years, the base will be undisputed, legally secure, with full Mauritian backing.”

Asked why the Islands should not be seen as a precedent for other sovereignty disputes such as the Falklands and Gibraltar, the spokesman said: “It’s a unique situation based on its unique history and circumstances, and has no bearing on other overseas territories.”

The spokesman added: “British sovereignty of the Falkland Islands or Gibraltar is not up for negotiation.”