The US has backed the UK's deal to hand over the Chagos Islands to Mauritius and lease back a key military base, Downing Street has told the BBC.

On Thursday, Donald Trump signalled his approval for the move, describing Sir Keir Starmer's agreement as the "best he could make".

It comes just a few weeks after the US president prompted fears in Whitehall that he would withdraw his support, after he branded the deal an "act of great stupidity".

Trump's comments led to additional talks between officials to confirm continued American support for the agreement, which will impact the future of a joint UK-US airbase.

The deal, originally announced last year, would see the UK transfer sovereignty of the Chagos Islands while leasing back the joint military base on the largest island, Diego Garcia, for an initial 99 years.

During a conversation on Thursday, the two leaders "agreed on the importance" of the deal to secure the base, Downing Street said.

A spokeswoman said they agreed the UK and US would "continue to work closely on the implementation of the deal".

In a post on his Truth Social platform earlier on Thursday, Trump wrote that his discussions with Sir Keir had been "very productive".

He added: "I understand that the deal Prime Minister Starmer has made, according to many, the best he could make."

"However, if the lease deal, sometime in the future, ever falls apart, or anyone threatens or endangers US operations and forces at our base, I retain the right to militarily secure and reinforce the American presence in Diego Garcia," he said.

Asked about Trump's comments, White House Press Secretary Karoline Leavitt told reporters: "He spoke with Prime Minister Starmer directly, he understands Prime Minister Starmer's position - and he supports it."

"But as the president reiterated in that statement, of course, the United States reserves the right to protect our assets."

Warren Stephens, the US's ambassador to the UK, said while the "ideal" outcome would be for the UK not to transfer sovereignty of the Chagos Islands, it was the "best deal on the table for successive UK governments".

Echoing the president, Stephens said the US "retains the right to maintain and to reinforce our security interests on Diego Garcia if needed in the future".

The deal was originally announced last year, and had been endorsed by Trump and US officials. But last month the US president labelled it an "act of total weakness".

He added in a post on Truth Social: "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."

Trump's criticism of the agreement came during an international row over his threats to take control of Greenland.

At the time, Sir Keir accused Trump of making the comments to pressure the UK over its support for the sovereignty of the Danish territory.

The comments threw into doubt whether the US would continue to support the Chagos deal. At the time, Downing Street said it believed the US still supported it, despite the president's comments.

Officials in London and Washington have since been involved in talks to determine whether US support for the agreement still stood.

The BBC understands those talks, which included two direct phone calls between Starmer and Trump, have now been completed.

A draft law to ratify the Chagos Islands deal is making its way through Parliament but has been delayed since the president's outburst last month.

Sir Keir has insisted the deal is necessary to protect the continued operation of the base, amid previous attempts from Mauritius to dispute the legality of British sovereignty over the islands.

The deal has been heavily criticised by the Conservatives and Reform UK, who argue it undermines national security because of Mauritius's ties to China.

Conservative shadow foreign secretary, Dame Priti Patel, said Trump's statement "recognises a critical weakness in the surrender deal" about the lease arrangements.

"The Conservative Party's view is unchanged," she added.

"We have led the fight against this appalling surrender and we will continue fighting it to the end."