Boris Johnson has accused ministers of preparing to make “a colossal mistake” by handing over a strategic string of islands in the Indian Ocean to an ally of China.
The former prime minister said he had heard from “sources” that the Foreign Office is ready to sign a deal to transfer ownership of the Chagos Archipelago to Mauritius.
Writing in the Daily Mail he said the decision would “cast doubt on a major Western strategic asset” and risk undermining the UK’s strong military ties with the US.
The islands are crucial as home to the Diego Garcia military base, which is leased to America and from where jets flew during the Iraq and Afghanistan wars.
Government sources insisted there was no question of control over the strategic airfield being relinquished as part of any deal to hand over wider control.
Mr Johnson wrote: “If my sources are correct, we are about to hand our title away and for no good reason. It seems this country is on the verge of a colossal mistake.
“Just as the Chinese are building runways over every reef and atoll they can find – places that have never been Chinese possessions – we are throwing in the sponge.
“We are about to haul down the flag, casting doubt on a major Western strategic asset.”
Deal could harm relations with US
Mr Johnson said that the decision could harm relations with the US, which might then strike a direct deal with Mauritius over continued use of the islands.
“The Americans don’t give us crucial nuclear secrets just because they love little old England. They don’t share intelligence because they adore our quaint accents.
“We have a great and indispensable relationship because we have important things to offer including Diego Garcia. I can’t really believe that we are going to allow it to happen.”
James Cleverly, the Foreign Secretary, has previously stressed that he was working closely with US allies on the potential handover of the islands.
Speaking in July, he said: “They know that I understand absolutely how important the operations on Diego Garcia are, and they know that we will absolutely protect them.
“I made this point directly to the United States that we absolutely take the ability to operate from Diego Garcia as an absolute key element in this, and will absolutely ensure that we will be able to continue to do that in an unfettered way.”
Ten-year campaign by Mauritius
Ministers opened negotiations over the potential transfer of the Chagos Islands last November after a 10-year campaign by Mauritius.
Any agreement to hand them over is intended to allow former inhabitants, who were displaced in the 1960s when the airbase was built, to return.
The Telegraph understands that preparatory work for the talks first began earlier in 2022 while Mr Johnson was still the prime minister.
There have since been five rounds of negotiations with a sixth set to take place later this month. It is not thought that a deal is imminent.
A Foreign Office spokesman said: “The UK and Mauritius have held five rounds of constructive negotiations on the exercise of sovereignty over the British Indian Ocean Territory/Chagos Archipelago, and officials will meet again soon to continue negotiations.
“The UK and Mauritius have reiterated that any agreement between our two countries will ensure the continued effective operation of the joint UK-US military base on Diego Garcia, which plays a vital role in regional and global security.”