Where are the Chagos Islands?

Downing Street insisted the deal, which would see Britain give up sovereignty of the island territory and lease back a crucial military base there, is the “right thing”.

A hearing is expected to take place at 10.30am on Thursday and follows Mr Justice Goose granting an injunction at 2.25am against the Foreign Office for “interim relief” to Bertrice Pompe.

She is one of two British women born on the Chagos Islands who had previously taken legal action over the deal.

“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,” Mr Justice Goose said in his order.

The last-minute hurdle is the latest stumbling block to Sir Keir Starmer’s efforts to get the deal over the line.

The prime minister had been expected to attend a virtual ceremony alongside representatives from the Mauritian government on Thursday morning to sign off on the deal after several weeks in which it appeared to be on hold over political difficulties.

Under the injunction, the Government is required to “maintain the jurisdiction of the United Kingdom over the British Indian Ocean Territory until further order”.

The judge granted the injunction after considering an out of hours application from Ms Pompe and after reading a response from the Foreign Secretary and Prime Minister’s office.

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.”

Here is what it means.

Where are the Chagos Islands?

The British Indian Ocean Territory (BIOT) or Chagos Islands (formerly the Oil Islands) is an overseas territory of the United Kingdom situated in the Indian Ocean, halfway between Africa and Indonesia.

The Chagos are formed of 60 islands and Diego Garcia is where the airstrip is found. It is a key location for foreign policy and why the UK is keen to hang onto the use of the land.

The UK has had control of the islands since 1965 and a joint airbase with the US was established between then and 1973.

Many of the 4,000 strong Chagossian population have campaigned for it to come under the sovereignty of Mauritius, another island collective that is off the east coast of Africa. The distance between the two is still a lengthy 1,300 plus miles and the Chagos Islands are nearer to the Maldives.

Sir Keir announced in October that Britain was prepared to hand over control to Mauritius but would continue to lease the air strip on Diego Garcia.

A Foreign Office spokesperson said: "We are working to sign a deal which will safeguard the joint UK-US base on Diego Garcia, which is vital to our national security."

Critics of proposals to hand over the islands to Mauritius fear the move will benefit China, which has a growing reach in the region.

The Conservatives are among those which have criticised Labour’s handling of the negotiations, though they began discussing the handover with Mauritius when they were in power.

Shadow justice secretary Robert Jenrick said: “If this group can force the Government to think twice, then all power to them.”