Mauritius could ban the US from having nuclear-capable weapons and vehicles on the Chagos Islands if Labour's surrender deal goes ahead.
The country previously signed onto the African nuclear weapon-free zone treaty, which prevents the "stockpiling, acquisition, testing, possession, control or stationing of nuclear weapons".
If the ban were to be lifted, it would require all 50 signatories to approve, which could take years to negotiate.
On Monday, the Conservatives said the deal "would expressly prohibit" Mauritius from allowing the US to have "any nuclear explosive device at the Diego Garcia military base".
It is likely to pose another difficult obstacle for Keir Starmer to hurdle to get the deal to cede the territory over the line.
While Donald Trump branded it an "act of great stupidity" on social media last week, the effects on the US's use of the Diego Garcia military base are likely to add to a list of concerns.
They have previously conducted exercises in the region with the nuclear-capable bomber B-52 jets - these could be banned from use at the base.
But the Government has repeatedly insisted the nuclear weapons treaty will not affect operations at Diego Garcia.
Mauritius could ban the US from having nuclear-capable weapons and vehicles on the Chagos Islands if Labour's deal goes ahead
| GETTYStephen Doughty, the Foreign Office minister, previously said: "We are confident that nothing in this treaty conflicts with our abilities to uphold international law, and to continue to operate the base as we do today."
Despite this, Prof Richard Ekins KC, a constitutional law expert and the head of Policy Exchange’s Judicial Power Project, said the Pelindaba treaty would apply if the handover goes ahead.
Speaking to The Telegraph, he said: "If the UK cedes the Chagos Islands to Mauritius then Diego Garcia will be Mauritian territory and the terms of the treaty of Pelindaba will apply.
"That treaty requires Mauritius to prohibit the stationing of nuclear weapons on its territory. The UK government may expect Mauritius to turn a blind eye but this is clearly storing up future trouble."
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Donald Trump has been public in his criticism of the Chagos Island deal
| PALast week, the Government admitted the President has the legal power to veto the deal, after conceding that an overhaul to the 1966 UK-US defence deal would be required before passing the controversial legislation.
In response to a letter from Lord Callanan, Baroness Chapman confirmed a deal can "only be ratified when the relevant domestic law and international arrangements in place".
Nigel Farage also warned that President Trump "won't put up with" Labour's Chagos deal after he spoke with US Treasury Secretary Scott Bessent during the Davos conference last week.
Speaking in the Commons on Monday, Richard Tice, the deputy leader of Reform, said: "Thank the heavens the US administration has now realised that they were deliberately misled by our national security adviser, deliberately misled by the Foreign Office.
"The national security adviser deliberately misled the American administration. And they are angry. They are furious, minister, at what has gone on, and that is why, that is why the American administration has changed their tune."
Sir Keir was forced to withdraw his Chagos Island Bill from the House of Lords on Friday after the Conservatives warned it could violate a US-UK treaty from 1966.
The deal is now expected to collapse should President Trump refuse the necessary amendments to the treaty.
The proposed arrangement sees Britain returning the archipelago to Mauritius while securing continued use of Diego Garcia, which houses a joint UK-American military base, through a century-long lease valued at over £30billion.