Keir Starmer's deal to hand the Chagos Islands to Mauritius is under threat after the island nation's new government said the agreement wasn't good enough.
Navin Ramgoolam said the draft deal "would not produce the benefits that the nation could expect" and negotiations had restarted.
He said the new Mauritian administration has submitted its own suggestions to the UK, which has now responded with its counter-proposals.
The UK struck a deal with Mr Ramgoolam's predecessor to cede sovereignty over the islands – also known as the British Indian Ocean Territory – but lease back the strategically important UK-US military base on Diego Garcia.
But Mr Ramgoolam told his country's parliament on Tuesday that during discussions with a delegation of UK officials last week "Mauritius made clear that while it is still willing to conclude an agreement with the United Kingdom, the draft agreement which was shown to us after the general elections is one which, in our view, would not produce the benefits that the nation could expect from such an agreement".
Former Defence Secretary Sir Michael Fallon condemns giving up the Chagos Islands
"Therefore, Mauritius accordingly submitted counter-proposals to the UK so that an agreement which is in the best interest of Mauritius can be concluded.
"The response of the United Kingdom to our counter-proposals was received yesterday afternoon and is now being currently considered."
Senior figures in the incoming Trump administration have also voiced doubts over the agreement, which is aimed at securing the legal basis for the Diego Garcia facility.
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.
Nick reacts to Labour's decision to hand the Chagos Islands back to Mauritius
But Downing Street insisted the deal was necessary to resolve a long-standing legal dispute over sovereignty.
The Prime Minister's official spokesman said the UK had been engaging with the new Mauritian government on the deal.
"The agreement we've struck with Mauritius protects the long-term secure operation of the UK-US base, which plays such a crucial role in regional and international security," the spokesman said.
"There's obviously been a change of leadership in Mauritius, and we've been engaging with the new administration on the details of the deal.
"We remain confident that the agreement is in both sides' shared interest."
Asked whether the UK had offered more money to the Mauritians for the 99-year lease on Diego Garcia, the spokesman said: "I'm obviously not going to get into the detail of private conversations."
The Conservatives sent a letter to Sir Keir saying they have "grave concerns" about the deal and warned against "rushing it through" Parliament before the Trump administration enters the White House.
Dame Priti Patel and James Cartlidge, the shadow foreign and defence secretaries, said the deal was "wholly unsustainable" given the change of US administration and the "rejection" of the proposed deal by the new Mauritian government.
They wrote: "If the Government has any inclination to try and salvage this deal by rushing it through Parliament before the new US administration takes office, it should think again.
"At a very minimum you should wait for the new administration in the US to take office – but our view remains that the legal justification you have used for giving away sovereignty is flawed, and it is therefore no surprise that the terms reached are so disadvantageous to the United Kingdom."