The government has temporarily been banned from concluding talks on the Chagos Islands deal by a last-minute injunction by a High Court judge.
At 02:25 BST Mr Justice Goose granted “interim relief” to two Chagossian women who had brought a case against the Foreign Office.
Prime Minister Sir Keir Starmer had been expected to attend a virtual signing ceremony with representatives of the Mauritian government on Thursday morning.
The agreement would see the UK hand the Chagos Islands to Mauritius, but allow the UK and US to continue using a military base located on the Pacific Ocean archipelago for an initial period of 99 years.
The UK government is yet to set out the estimated payments the British taxpayer would make to Mauritius as part of the deal, but it is expected to run into the billions.
Responding to the court injunction, a government spokesperson said: “We do not comment on ongoing legal cases. This deal is the right thing to protect the British people and our national security.”
The court will hear the case again at 10:30 on Thursday.
The legal action was brought by two Chagossian women, Bernadette Dugasse and Bertrie Pompe.
Earlier this year, their lawyer Michael Polak said: “The government’s attempt to give away the Chagossians’ homeland whilst failing to hold a formal consultation with the Chagossian people is a continuation of their terrible treatment by the authorities in the past.
“They remain the people with the closest connection to the islands, but their needs and wishes are being ignored.”
In his early-morning order, the judge said: “The defendant shall take no conclusive or legally binding step to conclude its negotiations concerning the possible transfer of the British Indian Ocean Territory, also known as the Chagos Archipelago, to a foreign government or bind itself as to the particular terms of any such transfer.”
“The defendant is to maintain the jurisdiction of the United Kingdom over the British Indian Ocean Territory until further order.”
The order states that the judge granted the injunction “upon consideration of the claimant’s application for interim relief made out of court hours” and “upon reading the defendants’ response”.
The Chagos Archipelago was separated from Mauritius in 1965, when Mauritius was still a British colony.
Britain purchased the islands for £3m, but Mauritius has argued it was illegally forced to give away the islands in order to get independence from Britain.
In the late 1960s Britain invited the US to build a military base on Diego Garcia and removed thousands of people from their homes on the island.
An immigration order, issued in 1971, prevented the islanders from returning.
In recent years, the UK has come under growing international pressure to return the islands to Mauritius, with both the United Nations’ top court and general assembly siding with Mauritius over sovereignty claims.
In late 2022, the previous Conservative government began negotiations over control of the territory but did not reach an agreement by the time it lost power in the 2024 general election.
The Conservatives have been strongly critical of the government’s deal.
In February, shadow foreign secretary Priti Patel called the agreement “botched and humiliating” accusing the government of “giving away one of the most important strategic military assets that we hold”.
Reform UK’s Nigel Farage said the UK would be paying “eye-watering” sums to Mauritius and suggested the islands should be handed to the US instead.
