Trump fired off a series of tweets about Ambassador Kim Darroch hours after British Prime Minister Theresa May gave the veteran diplomat her continued support.
"The wacky Ambassador that the U.K. foisted upon the United States is not someone we are thrilled with, a very stupid guy," Trump wrote in one tweet.
Darroch's forthright, unfiltered views on the U.S. administration — meant for a limited audience and discreet review — appeared in leaked diplomatic documents that were published in Britain's Mail on Sunday newspaper.
The disclosures have caused embarrassment and an awkward situation for two countries that often celebrate having a "special relationship."
In his Twitter comments Tuesday, Trump combined criticism of Darroch with a broadside at May, chiding the British leader for failing to get her Brexit deal with the European Union through Parliament.
"I told @theresa—may how to do that deal, but she went her own foolish way-was unable to get it done. A disaster!" Trump tweeted. "I don't know the Ambassador but have been told he is a pompous fool."
Darroch has served as Britain's envoy in Washington since 2016. In one of his leaked memos, he said that to communicate effectively with Trump, "you need to make your points simple, even blunt."
The published documents also included the ambassador calling the Trump administration's policies on Iran "incoherent," saying the U.S. president might be indebted to "dodgy Russians," and raising doubts about whether the Trump White House "will ever look competent."
Darroch has had a close relationship with numerous Trump administration officials. The president's advisers have been frequent guests at British Embassy events.
An investigation is underway to find who was responsible for leaking the memos, a major breach of diplomatic security.
May's spokesman said Tuesday that the prime minister phoned Darroch to tell him he still had her full support.
But the tweets by Trump, which followed a similar social media barrage on Monday, ratcheted up pressure on Britain's government. Darroch also has been accused by some Brexit-backing U.K. politicians of lacking enthusiasm for Britain's departure from the European Union.
The journalist who reported the leak, Isabel Oakeshott, is a strong Brexit backer and an ally of Brexit Party leader Nigel Farage, who also is Britain's leading champion of Trump.
Trump once said Farage would "do a great job" as ambassador to the United States. Farage sidestepped the idea Monday, saying "I'm not a diplomat."
The tiff with Trump also put pressure on Boris Johnson and Jeremy Hunt, the two men vying to succeed May as Conservative leader and prime minister. Both say they will lead the U.K. out of the European Union and secure new trade deals around the world — notably with the United States.
Hunt, who is Britain's current foreign secretary, reprimanded Trump on Tuesday, writing in his own tweets that the president's comments about Darroch were "disrespectful and wrong."
During a televised debate Tuesday night, Hunt said "if I am our next prime minister, the ambassador in Washington stays, because it is our decision."
Johnson declined during the debate to make a similar commitment to keep Darroch in his post, though he said whoever leaked the diplomatic cables should be "eviscerated."
"I think it's very important we should have a close partnership, a close friendship with the United States," he said.
While British officials hunted for the culprit behind the leak, senior Conservative Party figure and former Foreign Secretary William Hague said the government was right to back Darroch.
"You can't change an ambassador at the demand of a host country," Hague told the BBC. "It is their job to give an honest assessment of what is happening in that country."
Associated Press writers Julie Pace in Washington and Jill Lawless in London contributed to this story.