Donald Trump has said he's "disappointed with Keir [Starmer]" over the prime minister's initial refusal to send aircraft carriers to the Middle East.
Repeating his jibe from a few days into the Iran war, the US president said: "Unfortunately Keir is no Winston Churchill."
Mr Trump was speaking to reporters in the Oval Office alongside Irish Taoiseach (prime minister) Micheál Martin to mark St Patrick's Day.
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He was asked whether his relationship with the UK had been damaged by Sir Keir Starmer's refusal to fully support the US's offensive military action against Iran.
Mr Trump replied: "Well, he hasn't been supportive, and I think it's a big mistake. You know, they make a lot of money on trade with the United States. I went out of my way [to make a trade deal]."
"We made a good deal for them and, frankly, probably wasn't appreciated."
The US and UK announced an economic prosperity deal last year, although the UK is still subject to a 10% baseline trade tariff from the US, which is higher in certain sectors.
The US president said that the US's relationship with the UK was "always the best until Keir came along".
"I like him... he's a very nice man with a beautiful family, everything's perfect," he said.
Mr Trump said he was "disappointed because Keir was willing to send two aircraft carriers after we won because essentially, there's no threat... [and we] said, no, no, we want things sent before the war, not after the war is won."
Downing Street sources denied that the US requested the UK send aircraft carriers to the Gulf.
Mr Trump said later: "I just said 'you don't have to send me aircraft carriers, send me a couple of minesweepers, we don't need your aircraft carriers', but even the aircraft carriers, he would only send them after we essentially won.
"He's sending them when there are no planes left, when the missiles are down to 8%."
It follows Mr Trump saying on Monday that the UK was previously considered the "Rolls-Royce of allies".
Defending Sir Keir, Mr Martin said the UK prime minister was a "very earnest, sound person" who Mr Trump has "a capacity to get on with and you've got on with before".
Trump criticises energy and immigration policies
The US president also criticised some of Sir Keir's policies outside the war - but said he did it "in a friendly way".
Mr Trump said: "I disagree with [Sir Keir] on two things.
"Primarily, immigration. His immigration policy is a disaster, and his energy policy is a disaster. And they are about the biggest policies you can have."
He said the UK needs to "get away from windmills and go back to oil and gas", suggesting exploiting reserves in the North Sea.
While he added that Sir Keir had "allowed millions and millions and millions of people to come into [the UK] that shouldn't be there".
The US president has criticised Sir Keir on both areas previously.
Asked whether he had confidence in Sir Keir, Mr Trump said it was a matter for "the people of the UK".
Sky News has asked Downing Street for a response to Mr Trump's comments.
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