Donald Trump has excoriated the UK's ambassador to Washington, saying he will "no longer deal with" the beleaguered diplomat.
The US president also turned his fire on Theresa May, accusing her of making a "mess" of Brexit.
Mr Trump added that the UK was "wonderful," and the "good news" was that it would soon have a new leader.
His intervention came after leaked diplomatic cables showed Sir Kim Darroch, the UK's top diplomat in Washington, had described Mr Trump's administration as "inept" and "incompetent".
Sir Kim was reportedly due on Monday night to attend a dinner at the White House, in honour of the Emir of Qatar, but had been "disinvited" in the wake of the leaks.
Writing on Twitter, Mr Trump said: "I have been very critical about the way the UK and Prime Minister Theresa May handled Brexit. What a mess she and her representatives have created. I told her how it should be done, but she decided to go another way.
I have been very critical about the way the U.K. and Prime Minister Theresa May handled Brexit. What a mess she and her representatives have created. I told her how it should be done, but she decided to go another way. I do not know the Ambassador, but he is not liked or well....— Donald J. Trump (@realDonaldTrump) July 8, 2019
....thought of within the U.S. We will no longer deal with him. The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister. While I thoroughly enjoyed the magnificent State Visit last month, it was the Queen who I was most impressed with!— Donald J. Trump (@realDonaldTrump) July 8, 2019
"I do not know the Ambassador, but he is not liked or well thought of within the US. We will no longer deal with him. The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister."
Mr Trump also praised the Queen, adding: "While I thoroughly enjoyed the magnificent State Visit last month, it was the Queen who I was most impressed with!"
The president targeted Mrs May on Twitter hours after the prime minister said she had “full faith” in Sir Kim.
Mrs May’s spokesman said it was Sir Kim’s role to provide "an honest and unvarnished view" of Mr Trump’s administration, although British officials have been forced to apologise.
Aides close to the president told The Telegraph at the weekend that they wanted him sacked, suggesting that he was too pro-Europe.
Meanwhile, it emerged that the Government has no idea how many classified diplomatic cables written by Sir Kim had fallen "into the wrong hands."
The leak prompted a demand for the Metropolitan Police, aided by the security services, to begin a criminal inquiry into the source.
The hunt for the mole who leaked the cables, some of which were published in a Sunday newspaper, is being led by the Cabinet Office with power to comb through phone records and emails of senior ministers who had received the memos.
Sir Alan Duncan, the senior Foreign Office minister, told MPs on Monday: “In terms of the emails that have been leaked they are two years apart - one cluster very recent and one of two years ago.
“But of course we don’t know if there are any others in the wrong hands which might subsequently be leaked.”
MI5, MI6 and GCHQ could also assist the inquiry, Sir Alan suggested.
He said that the Cabinet Office would “use all of the means it can to delve into this and try to find a culprit” and added that the mole, if discovered, would “regret the moment for the rest of their life”.
Sir Alan conceded that the number of recipients of Sir Kim’s emails “well exceeds” 100 recipients.
The Conservative MP Tom Tugendhat, the chairman of the Foreign Affairs Committee, announced he had written to Cressida Dick, the Metropolitan Police commissioner, formally requesting her force begin a criminal inquiry.
Sir Simon McDonald, Permanent Secretary at the Foreign Office, told the Public Accounts Committee that “significant damage” had been caused by the leak.
Asked if Mrs May agreed with the contents of Sir Kim’s leaked assessment of the Trump administration, the spokesman said: “The PM does not agree with that assessment.”
Earlier in the day, International Trade Secretary Liam Fox on a visit to the US that includes a meeting with Mr Trump’s daughter Ivanka, said: “This is such a damaging, potentially damaging, event that I hope the full force of our internal discipline, or even the law, will come down on whoever actually carried out this particular act.”
Jeremy Hunt, the Foreign Secretary, distanced himself from Sir Kim’s remarks telling a press conference: “It’s a personal view and there will be many people in this building who don’t agree with that view and indeed I don’t agree with some of the views that we saw in those letters.
“I think the US administration is highly effective and we have the warmest of relationships and a partnership based on standing up for shared values.”
It is the second major leak inquiry in Government this year. Gavin Williamson was sacked as defence secretary after being accused by Mrs May of being the source of a Telegraph story that detailed a row over the involvement of Huawei, the Chinese telecoms company, in helping to build Britain’s 5G network.