Boris Johnson has called Rishi Sunak “spineless” over the decision to hand the strategically important Chagos Islands over to an ally of China.
Part of the British Indian Ocean Territory since 1814, the region which is also called Archipelago is home to a large British-American military base, Diego Garcia.
Mr Johnson said that the islands near the Maldives are a “key component” of US-UK relations and are crucial for our mutual strategic interests.
Washington is concerned by the growing relationship between Mauritius and China with the Pentagon fearing Beijing is looking for a military foothold in the region.
The ex-PM said in his Daily Mail column: “It seems that this country is on the verge of a colossal mistake. After more than two centuries of uninterrupted British sovereignty, we are apparently about to perform a U-turn and abandon the British Indian Ocean Territories.
“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.”
He also disputed Mauritius’s claim to the islands, saying that although Britain used to administer them together they are 1,300 miles apart and the islands have never been part of Mauritius.
The Government has previously rejected Mauritian claims to the Chagos Islands but Foreign Secretary James Cleverly has agreed to broker an agreement to “resolve all issues” while keeping the base operational.
When the military base was built on Diego Garcia, just under 1,000 people were kicked off of the island – the descendants of African slaves brought by the French. Mr Johnson called their eviction “painful” but necessary.
The base played a crucial role in the 1990 Gulf War, the Afghanistan war in 2001 and the Iraq war in 2003.
