The British government has not given permission for the US to use UK military bases to support potential US strikes on Iran, the
BBC understands.
The US has in the past used RAF Fairford, in Gloucestershire, and the UK overseas territory of Diego Garcia, in the Indian Ocean, to carry out strikes in the Middle East region.
The Times reports that US President Donald Trump's latest criticism of Sir Keir Starmer's deal to hand the Chagos Islands over to Mauritius and lease back a joint UK-US military base was prompted by the UK withholding permission to use bases.
A UK government spokesperson said: "As routine, we do not comment on operational matters."
The US has been pressuring Iran to agree to curb its nuclear programme.
It has threatened possible strikes if it fails to do so and has moved warships, aircraft and other military assets to the region in preparation for a possible strike.
At the same time, some progress has been reported at talks between American and Iranian negotiators in Switzerland.
On Thursday, Trump said the world would find out "over the next, probably, 10 days" whether the US would reach a deal with Iran or take military action.
A UK government spokesperson said: "There is a political process ongoing between the US and Iran, which the UK supports.
"Iran must never be able to develop a nuclear weapon, and our priority is security in the region."
Sir Keir spoke to Trump over the phone on Tuesday, after the US government gave its formal support for the Chagos deal.
The Times reports that the pair discussed Trump's ultimatum to Iran over its nuclear programme, and Trump made his statement attacking the Chagos deal the next day.
Insiders believe the Pentagon's preparations for possible strikes on Iran - which could potentially be launched from Diego Garcia - may have shifted the president's understanding of the significance of the island, the largest in the Chagos archipelago.
Trump referenced this in his Truth Social post, saying: "Should Iran decide not to make a deal, it may be necessary for the United States to use Diego Garcia, and the [RAF] Airfield location in Fairford [Gloucestershire], in order to eradicate a potential attack by a highly unstable and dangerous regime."
Neither RAF Fairford nor Diego Garcia were used in the US strikes carried out last year on Iran's nuclear facilities.
At the time a senior UK defence source said Washington had not sought permission.
