The government has been temporarily blocked from concluding the Chagos Islands deal by a late-night High Court injunction.
Ministers had been expected to complete a deal that would have seen the UK hand over sovereignty of the archipelago to Mauritius in the coming hours.
But in an emergency injunction granted early on Thursday, brought against the Foreign Office, Mr Justice Goose allowed “interim relief” to Bertrice Pompe, who had previously taken steps to bring legal action over the deal.
Ms Pompe is a Chagossian woman who sees the deal as a betrayal of their rights.
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The order, granted at 2.25am, states the government may 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”.
A hearing is expected to take place at the High Court at 10.30am on Thursday.
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The government has insisted this morning that the Chagos Islands deal is the “right thing” for the UK.
A 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.”
It was expected that Sir Keir Starmer would attend a virtual ceremony today to formally hand over sovereignty of the Chagos Islands to Mauritius, despite heavy criticism from the Conservatives and Reform UK.
The government has argued that international legal rulings in favour of Mauritius mean this handover is necessary.
As part of the deal, the UK will lease back a military base on the archipelago for 99 years.
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Robert Jenrick, the former justice secretary, told Sky News that the Chagos Islands deal is a “sell-out for British interests”.
He said: “You’re seeing British sovereign territory being given away to an ally of China and billions of pounds of British taxpayers money being spent for the privilege.
“So, if this group can force the government to think twice, then all power to them.”
With this injunction in place, Sir Keir can no longer legally complete the deal.
Ms Pompe, who filed the application for interim relief, believes the British government is acting with disregard for the human rights of the Chagossian people.
She has argued that completion of the deal would amount to a breach of the Human Rights Act and the Equality Act.
Chagossians are the former residents of the Chagos Islands, who were removed from the islands, predominantly to Mauritius, between the mid-1960s and early-1970s.
Those born on the islands and their children hold British nationality, but subsequent generations born outside British territory currently have no entitlement to it.
