Sir Keir Starmer faces being pushed for further concessions over his plan to cede sovereignty over the Chagos Islands, with the Mauritian government seeking another round of talks.
A Cabinet meeting called by Mauritian Prime Minister Navin Ramgoolam did not sign off on the proposed deal and are instead sending a delegation back to London for further negotiations.
The UK plans to hand over the British Indian Ocean Territory to Mauritius then lease back the strategically important military base on Diego Garcia, which is used by the US, at a reported cost of £90 million a year.
The Government argues that international court rulings in favour of Mauritian sovereignty mean that a treaty settling the future of the archipelago is the only way to guarantee the continued operation of the base.
The Labour administration concluded a deal with Mr Ramgoolam’s predecessor, but the change of government in Mauritius and Donald Trump’s election in the US have stalled progress.
Sir Keir defended his plan at Prime Minister’s Questions, insisting it was the best way to safeguard the military base.
In response to criticism from Tory leader Kemi Badenoch, he said: “We inherited a situation where the long-term operation of a vital military base was under threat because of legal challenge.
“The negotiations were started under the last government. The then foreign secretary came to this House to say why he was starting negotiations and what he wanted to achieve. He said the aim was to ‘ensure the continued effective operation of the base’.
“That is precisely what this deal has delivered.”
Mrs Badenoch said Sir Keir was “negotiating a secret deal to surrender British territory and taxpayers in this country will pay for the humiliation”.
A statement issued by the Mauritian government said the Cabinet had been “informed of developments” following attorney general Gavin Glover’s talks in London last week but had decided that negotiations should continue.
“The attorney general will travel again this evening in order to continue with the discussions in London.
“Upon his return, Cabinet will then be apprised of the situation.
“The commitment and resolve of Mauritius to reach an agreement and end this long battle for the sovereignty of Mauritius over the Chagos Archipelago remains unshaken.”
Mr Ramgoolam had previously told the Mauritian parliament that the UK wanted to conclude a deal before Mr Trump’s inauguration on January 20.
But the Financial Times reported the UK is considering delaying signing the deal to await Mr Trump’s blessing.
The president-elect’s pick for secretary of state, Marco Rubio, warned in October that the agreement posed “a serious threat” to US national security by handing over the islands to a country allied with China.
And Reform UK’s leader Nigel Farage, an ally of Mr Trump, has warned of “very deep disquiet” about the deal in the president-elect’s inner circle.