A furious row has erupted after the government announced that it has given up sovereignty over the Chagos Islands after more than 50 years.
Foreign secretary David Lammy said the move would “strengthen our role in safeguarding global security”.
But critics accused the government of “surrender” and said it put other contested territories, like Gibraltar and the Falkland Islands, under threat.
The Chagos Islands are strategically significant because of their location in the Indian Ocean and contain a UK-US military base at Diego Garcia.
Under the agreement, control of the islands will be handed over to the government of Mauritius - but the military base will continue to operate as it does at the moment.
Chagossians were forced to leave the central Indian Ocean territory by 1973 to make way for the military base at Diego Garcia.
The expulsions are regarded as one of the most shameful parts of Britain’s modern colonial history and Chagossians have spent decades fighting to return to the islands.
The International Court of Justice (ICJ) previously ruled the UK’s administration of the territory was “unlawful” and must end.
Lammy said: “This government inherited a situation where the long-term, secure operation of the Diego Garcia military base was under threat, with contested sovereignty and ongoing legal challenges.
“Today’s agreement secures this vital military base for the future. It will strengthen our role in safeguarding global security, shut down any possibility of the Indian Ocean being used as a dangerous illegal migration route to the UK, as well as guaranteeing our long-term relationship with Mauritius, a close Commonwealth partner.”
The move, which follows two years of negotiations between the UK and Mauritius, was also welcomed by US president Joe Biden.
He said: “I applaud the historic agreement and conclusion of the negotiations between the Republic of Mauritius and the United Kingdom on the status of the Chagos Archipelago.
“It is a clear demonstration that through diplomacy and partnership, countries can overcome long-standing historical challenges to reach peaceful and mutually beneficial outcomes.”
But Conservative Party leadership contenders condemned the deal in separate posts on X.
James Cleverly, who was foreign secretary when the negotiations between the UK and Mauritius began, said: “Weak, weak, weak! Labour lied to get into office. Said they’d be whiter than white, said they wouldn’t put up taxes, said they’d stand up to the EU, said that they be patriotic. All lies!”
Tom Tugendhat also criticised the move, and appeared to take a swipe at Cleverly by pointing out the negotiations with Mauritius began when Liz Truss was PM and he was foreign secretary.
“This deal not only threatens our security but also undermines our allies, opening the possibility of China gaining a military foothold in the Indian Ocean,” he said.
“Initiated during Liz Truss’ premiership, these negotiations should never have started, as the ICJ had no jurisdiction to challenge the UK’s sovereignty and its advisory opinion is non-binding under international law.
“It compromises both our national security and the stability of the region. Lord Cameron rightly intervened and stopped these negotiations, but Labour’s total failure of leadership and strategic vision has left our Indian Ocean allies vulnerable.”
Robert Jenrick said: “Handing over our strategically important territory to an ally of China and paying them for the privilege is an act of surrender.
“It has clearly been announced while Parliament isn’t sitting to avoid scrutiny but we will fight this dangerous sellout whenever Starmer brings legislation forward to enable it.”
The other leadership candidate, Kemi Badenoch, said: “Two days after Israel is attacked directly by Iran, Labour have put our vital base in the Indian Ocean at risk.
“Anyone who has been paying attention to the geopolitical climate will understand the strategic necessity of these islands. Labour has either not been paying attention or don’t care. Either way, this decision weakens UK influence across the world.”
But Tony Blair’s former chief staff Jonathan Powell, who helped negotiate the deal, hit back at its Tory critics.
He told Radio 4′s World At One programme: “I do think these comments from the Tory leadership candidates are a bit silly. James Cleverly was leading these negotiations not that many months ago with the Mauritian government.”
Powell also pointed out that the UK had lost legal challenges brought against the government by those living on the islands.
He said: “Legal advice was clear that we were not going to be able to sustain that position. So that previous government started a negotiation.
“What this government has done has been able to conclude it, because [the last] government managed to lose the trust of the Mauritian government during the process.
“So for the people who were involved in that negotiation to start criticising the outcome of something they couldn’t achieve is absolutely ludicrous.”