He described the move as “utterly spineless” and said the UK looked to be “on the verge of a colossal mistake”, calling Mauritius’s claim to the islands “preposterous”.
The UK entered negotiations with Port Louis over the future of the Chagos Islands after international pressure.
The United Nations’ highest court, the International Court of Justice, has ruled that the UK’s administration of the territory is “unlawful” and must end after two centuries of British control.
But the Conservative Government said that whatever the outcome of the talks between the two countries, a joint UK-US military base on Diego Garcia, one of those in the cluster of Indian Ocean islands that make up the Chagos Islands, will continue to be operated by the allies.
A future Mauritian government might close the base or allow the Chinese, at the right price, to build their own runways on the same archipelago
It comes after Mr Johnson said in his weekly column for the Daily Mail that he had been told by an “informant” that it is a “done deal” that the UK will give the Chagos Islands to the control of the Commonwealth nation.
Mr Johnson pointed out the significance one of the volcanic islands, Diego Garcia, plays in the relationship between Britain and the US, with it used as a base during the first and second Gulf wars, as well as the allied invasion of Afghanistan.
The former Conservative Party leader said: “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.
“Who knows what happens if we give up sovereignty. The Mauritians might cut out the middleman, and do a deal with the US (and never mind the Chagossians).
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
“But then a future Mauritian government might also close the base or allow the Chinese, at the right price, to build their own runways on the same archipelago.”
Foreign Office officials said the UK Government was working in “lockstep” with the US during the talks on the future of the Chagos Islands and said it had the “full support” of Washington on all elements of the negotiation.
The department said it would not be appropriate to speculate on the outcome of the talks while they were ongoing.
A spokesman for the Foreign, Commonwealth and Development Office 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.”
Chagossians have spent decades fighting to return to the islands after more than 1,000 people were forced to leave in the 1960s and 1970s to make way for the military base.
Thousands of Chagossians now live around the world, mostly in Mauritius, the UK and the Seychelles.
In a written ministerial statement published earlier this month, junior Foreign Office minister David Rutley said Prime Minister Rishi Sunak had met his Mauritian counterpart on the fringes of the G20 summit in New Delhi, India, to assess the progress of the negotiations.
Further negotiations are expected to take place this month, he said.