Britain's government has been temporarily banned from concluding the Chagos Islands deal with Mauritius - which include the future of a US-UK defence base - by an injunction granted by a High Court judge on Thursday.
Earlier reports said Britain was set to sign the deal to cede sovereignty of the Chagos Islands to Mauritius while also securing the future of the Diego Garcia military base in the Indian Ocean.
The deal, the details of which were first announced in October, would allow Britain to retain control of the base on Diego Garcia, the largest island of the archipelago in the Indian Ocean, under a 99-year lease.
Thursday's last-minute injunction was granted following action taken by Bernadette Dugasse and Bertrice Pompe, both British nationals who were born in Diego Garcia.
British Prime Minister Keir Starmer was due to join a virtual signing ceremony with representatives from the Mauritian government, the Telegraph newspaper reported earlier.
In 1965 Britain detached the Chagos Islands from Mauritius - a former colony that became independent three years later - to create the British Indian Ocean Territory.
Financial details of the deal have not been set out. Media reports have put the cost to Britain at 9 billion pounds.
U.S. President Donald Trump, who took office in November, indicated his backing for the deal in February after meeting Starmer in Washington. Trump's predecessor Joe Biden had also supported the agreement.