The UK government has insisted Mauritius is ready to finalise a deal over the future of the Chagos Islands, despite the country's new PM demanding better terms.
Under the agreement announced in October, the UK would hand sovereignty over the islands to Mauritius, but maintain a 99-year lease over the UK-US military airbase on the largest island, Diego Garcia.
Shadow foreign secretary Dame Priti Patel told MPs that it was a "monumental failure of statecraft", while Reform UK leader Nigel Farage said there was "very deep disquiet" about it among US President-elect Donald Trump's incoming team.
Foreign Office Minister Stephen Doughty argued the deal was in all sides' interests.
On Tuesday, Mauritius said it had put forward changes to the original deal, which was struck shortly before then-Prime Minister Pravind Jugnauth suffered a landslide defeat in a general election.
His successor Navin Ramgoolam told the Mauritian parliament he was "still willing to conclude an agreement", but the draft deal was not good enough for his country.
He did not spell out what changes he was seeking.
In the Commons, Conservative Dame Priti accused the Labour government of putting the UK's national security at risk, ignoring the interests of Chagossians, and "letting our standing go into freefall" in an increasingly dangerous world.
"If the deal is so good, why has the government been so secretive about the details?" she asked.
"Will we and the US still have full autonomy of operation? What safeguards will be in place to stop other countries, including China, from potentially trying to establish themselves on the base or near the military base on Diego Garcia?
"How much is the British taxpayer going to be liable for each year, and in total, over 99 years?" she also asked.
Doughty insisted the deal would enhance, not damage UK security, saying it would protect the military base's operation and ensure it was "on a safe footing well into the next century".
He said the UK would be able to extend its lease beyond 99 years, that Britain and its allies would continue to have operational autonomy, and that "safeguards" were in place to prevent other countries' forces operating on the archipelago's outer islands.
He said it was "not normal practice" to comment on the costs of military bases.
Doughty told MPs it was "completely understandable" that the new Mauritian government wanted time to study the details of the agreement, adding he was confident "we have agreed a good and fair deal that is in both sides' interests".
"It protects the base at proportionate cost. It has been supported across the national security architecture in the United States and by India," he said.
However, having just returned from visiting Trump's Mar-a-Lago resort for talks with Elon Musk and other allies of the president-elect, Farage challenged that.
"There is very deep disquiet amongst all of them as to what this may mean for the long-term future of Diego Garcia and whether such deal would hold, given the precedent of the deal break over Hong Kong," he told MPs.
The Reform UK leader also suggested the terms of the deal should be put to a referendum of exiled Chagossians.
Last month, Farage predicted that the deal would be met with "outright hostility" by the Trump administration, which takes office on 20 January.
He suggested Trump's advisers were worried it could boost China's influence in the region.
But, on Wednesday, Doughty said Farage did not know the details of the agreement, which US officials had also considered.
No 10 said it was not aware of any contact with the incoming Trump administration about the Chagos Islands deal.
"The focus of our discussions at the moment are obviously with the new Mauritian government," the prime minister's official spokesman said.
In recent years, the UK has faced increasing diplomatic isolation over the islands, with various United Nations bodies - including the International Court of Justice and the General Assembly - overwhelmingly siding with Mauritius's claim to sovereignty.
Mauritius has long argued it was illegally forced to give the islands away in return for its independence from the UK in 1968.
At the time, the British government had already negotiated a secret deal with the US, agreeing to lease it Diego Garcia for use as a military base.
The UK later apologised for forcibly removing more than 1,000 islanders from the entire archipelago and promised to hand the islands to Mauritius when they were no longer needed for strategic purposes.