"Do not give away Diego Garcia," US President Donald Trump has said as he criticised the UK's plan to hand over the Chagos Islands to Mauritius and lease back an important military base.

Trump said "this land should not be taken away from the UK" and if it did it would be "a blight on our Great Ally", in a post on social media.

This comes despite Washington on Tuesday giving its official backing to London's plan to cede sovereignty of the British Indian Ocean Territory to Mauritius.

In response to Trump, the UK Foreign Office said the Chagos Islands deal was "crucial to the security of the UK and our key allies, and to keeping the British people safe".

"The agreement we have reached is the only way to guarantee the long-term future of this vital military base," it said in a statement.

Trump's latest comments come ahead of talks between the US and Mauritius planned for next week.

Diego Garcia is the largest island in the archipelago, and is used as a joint military base by the UK and US armed forces.

Under the deal, announced last May, the UK will lease back Diego Garcia for 99 years.

Posting on the Truth Social platform on Wednesday, Trump wrote: "I have been telling Prime Minister Keir Starmer, of the United Kingdom, that Leases are no good when it comes to Countries, and that he is making a big mistake by entering a 100 Year Lease..."

The US president stressed that Diego Garcia was "strategically located in the Indian Ocean".

"Prime Minister Starmer is losing control of this important Island by claims of entities never known of before," he said.

"We will always be ready, willing, and able to fight for the U.K., but they have to remain strong in the face of Wokeism, and other problems put before them."

And referring to ongoing US-Iran talks on Tehran's controversial nuclear programme, Trump said: "Should Iran decide not to make a Deal, it may be necessary for the United States to use Diego Garcia... in order to eradicate a potential attack by a highly unstable and dangerous Regime."

The Republican president has repeatedly threatened to use military action against Iran over its deadly crackdown on anti-government protests and its nuclear activities.

The US and its European allies suspect that Iran is moving towards the development of a nuclear weapon, something that Tehran has always denied.

Asked by the BBC about Trump's latest comments, White House press secretary Karoline Leavitt said: "The post should be taken as the policy of the Trump administration, it's coming straight from the horse's mouth.

"When you see it on Truth Social, you know it's directly from President Trump; that's the beauty of this president in his transparency and relaying this administration's policies."

But on Tuesday, the US Department of State gave its official backing to the planned handover of the Chagos Islands.

In a statement, it said it "supports the decision of the United Kingdom to proceed with its agreement with Mauritius concerning the Chagos archipelago".

Trump's position on the issue has seen rapid U-turns in recent months - from describing the planned transfer as an "act of great stupidity" to saying the deal was the "best" the prime minister could make.

Shadow foreign secretary Dame Priti Patel said Trump's latest comments was "an utter humiliation" for Starmer.

"It's time Starmer finally saw sense, U-turned and scrapped this appalling deal altogether," she said.

Liberal Democrat leader Sir Ed Davey said Trump's latest change in position showed that the UK needed to pursue closer ties with Europe.

"Trump's endless flip-flopping on the Chagos Islands shows why Starmer's approach is doomed to fail," Davey wrote in a post on X.

"Britain can't rely on the US while Trump is in the White House. It's time to strengthen our ties with allies we can depend on, starting with our neighbours in Europe."

Last month, Reform leader Nigel Farage - an outspoken critic of the UK handover plan - praised Trump's earlier criticism of London's handling of the issue.

"Thank goodness Trump has vetoed the surrender of the Chagos islands," Farage wrote on X.

Earlier this week, four Chagos islanders - who are opposing the handover deal - landed on the Chagos Islands' remote atoll as part of a protest against the deal.

They refused to leave it, despite a threat of eviction by a British maritime patrol.