Donald Trump has watered down his criticism of the UK’s plan to hand the Chagos Islands back to Mauritius, saying the deal was the “best” Keir Starmer could make.

The US president had described ceding sovereignty of the British Indian Ocean Territory, which includes the Diego Garcia military base, as an “act of great stupidity” only last month. He also claimed the deal was one of many “national security reasons” why the US should acquire Greenland.

Under the deal, Britain would cede control over the islands to Mauritius but lease the largest, Diego Garcia, for 99 years to continue operating a joint US-UK military base there.

Trump endorsed the handover when Starmer visited the White House last year.

But posting on Truth Social last month, Trump wrote: “There is no doubt that China and Russia have noticed this act of total weakness.

“The UK giving away extremely important land is an act of GREAT STUPIDITY, and is another in a very long line of National Security reasons why Greenland has to be acquired.”

But after a “very productive” phone call with Starmer on Thursday, Trump appeared to backtrack to a position of approval for the deal.

The US president wrote on Truth Social that Diego Garcia was “strategically situated in the middle of the Indian Ocean and, therefore, of great importance to the National Security of the United States”.

He added that US military operations have succeeded over the past year “because of the strength of our warfighters, modern capability of our equipment and, very importantly, the strategic location of our Military Bases.

“I understand that the deal Prime Minister Starmer has made, according to many, the best he could make.”

But Trump added: “If the lease deal, sometime in the future, ever falls apart, or anyone threatens or endangers U.S. operations and forces at our Base, I retain the right to Militarily secure and reinforce the American presence in Diego Garcia.

“Let it be known that I will never allow our presence on a Base as important as this to ever be undermined or threatened by fake claims or environmental nonsense.”

After Starmer and Trump’s phone call, a Downing Street spokesperson said they “agreed on the importance of the deal to secure the joint UK-US base on Diego Garcia, which remains vital to shared security interests”.

The pair also agreed that the “UK and US will continue to work closely on the implementation of the deal”, the spokesperson said.

The Conservative party and Reform UK have been highly critical of the Chagos Islands deal. Reacting to Trump’s announcement, Priti Patel, the shadow foreign secretary, said: “The president’s statement today recognises a critical weakness in the surrender deal – the lease could fall apart, leaving our national security and that of our allies in even greater doubt. The deal is clearly a threat to our national security and is a massive strategic blunder.

“The Conservative party’s view is unchanged. We have led the fight against this appalling surrender and we will continue fighting it to the end.”

Trump’s reversal comes after he softened his plan to take control of Greenland, which is a semi-autonomous territory belonging to Denmark, a Nato member.

After talks with Nato’s secretary-general, Mark Rutte, on the sidelines of the World Economic Forum last month, Trump announced that a “framework of a future deal” on Greenland had been reached after weeks of escalating threats.