Ceding the Chagos Islands to Mauritius was “completely the wrong thing to do”, former prime minister Boris Johnson has warned.

His former colleague, shadow foreign secretary Andrew Mitchell, has also criticised the move as “not a deal” which previous Conservative foreign secretaries would have struck.

The United Nations’ highest court, the International Court of Justice, had previously ruled the UK’s administration of the Indian Ocean territory was “unlawful” and must end.

London agreed to hand over the Chagos Islands, also known as the British Indian Ocean Territory, to Mauritius in a decision unveiled on Thursday, which is subject to a treaty.

But former Tory prime minister Mr Johnson described the decision as “sheer political correctness”.

He told GB News on Sunday: “Why are we doing this? Sheer political correctness, a desire to look like the good guys, the desire to look as they were unbundling the last relics of our empire. It’s nonsense.

“It’s a bad idea in hard geopolitical terms because the (military) base in Diego Garcia is, I’m sure you know … of huge strategic importance for the US, for the West, and it’s a key component of the Anglo-American alliance.

“It’s one of the things we bring to the table and has been for decades, that base.

“Why are we trading away our sovereignty over Chagos? Completely the wrong thing to do.”

As part of the agreement, the UK and US will maintain a military presence on Diego Garcia for 99 years, with an option to renew.

Mr Mitchell suggested there is a question mark over “Chinese encroachment” into the archipelago which MPs must probe in the Commons.

He told Sky News’s Sunday Morning With Trevor Phillips programme: “This is definitely – from what we have seen – not the deal the last Conservative government would have done.”

Pressed on the Tories’ role in starting the Chagos talks in 2022, Mr Mitchell replied: “Starting the negotiations is not the same as concluding them, and from what I’ve seen this is definitely not a deal either (former foreign secretaries) David Cameron or indeed James Cleverly would have done, and we need to see the terms that they’re offering.

“We need to know about what protection there is against Chinese encroachment into the archipelago, we need to know what money is being provided by the British taxpayer. We need to probe the terms of the lease.”

The shadow foreign secretary added: “And we need to do all those things, and the right place to do them is in the House of Commons, and His Majesty’s Opposition will put down tomorrow (Monday) an urgent question in the House of Commons if there isn’t a statement from the Government so that we can probe the terms of this.

“But from what we have seen so far – and, by the way, it should’ve been announced in the House of Commons, which is the right place to do these things – from what we’ve seen so far, this looks like a bad deal for Britain.”

Labour’s Stephen Doughty, the Foreign Office minister responsible for the Overseas Territories, has not yet said how much the deal will cost British taxpayers.

“Full details will be set out in due course,” he has previously said.

“We inherited a completely unsustainable situation, and we’re confident that we’ve achieved a deal here that’s both legally sustainable, that reaches a settlement that’s financially sustainable and, crucially, protects the national security interests of this country, and indeed our allies.”