Donald Trump has dramatically escalated the transatlantic war of words over the leak of the British ambassador's advice describing his administration as "inept".
In an explosive series of tweets, the US president accused Theresa May of making a "mess" of Brexit.
He said the his administration would no longer deal with the ambassador, Sir Kim Darroch, while the "good news" for the UK was that it would soon have a new prime minister.
"I have been very critical about the way the UK and Prime Minister Theresa May handled Brexit," he said.
"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 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.
"While I thoroughly enjoyed the magnificent state visit last month, it was the Queen who I was most impressed with!"
Mr Trump's outspoken attack – the most strident public criticism of a British prime minister by a US president in decades – will alarm and dismay Downing Street.
It comes just a month after the Government rolled out the red carpet for the president for a state visit in which he praised the enduring strength of the "special relationship".
Earlier, the Prime Minister's official spokesman insisted that Mrs May had "full faith" in Sir Kim, while distancing No 10 from his assessment of the Trump White House.
"Our ambassadors provide honest, unvarnished assessments of politics in their country, those views are not necessarily the views of ministers or indeed of the Government," the spokesman said.
"This leak is not acceptable. We would expect such advice to be handled in the correct way and a leak inquiry has been launched."
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."
In the Commons, there were calls for the police to investigate amid widespread anger at the diplomatic fall-out from the leak.
Tom Tugendhat, chairman of the Commons Foreign Affairs Committee, told MPs he had written to the Metropolitan Police Commissioner Cressida Dick to "ask that a criminal investigation also be opened into the leak".
Foreign Office minister Sir Alan Duncan said police could be involved if
evidence of wrongdoing over the leak is found, telling the Commons: "If evidence of criminality is found, then yes, the police could be involved."
Earlier, Foreign Secretary Jeremy Hunt said he did not agree with all the views expressed by Sir Kim and insisted Britain still had the "warmest" of relationships with the US.
Speaking at a press conference at the Foreign Office, Mr Hunt said: "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."
However, International Trade Secretary Liam Fox – on a visit to the US – was less sanguine, expressing concern about the impact on US-UK relations after Mr Trump reacted angrily to the leak.
"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," he told the BBC.
The Foreign Office, which originally described the leak as "mischievous", launched a formal leak inquiry on Sunday.
In the memos, obtained by The Mail on Sunday, Sir Kim suggested that in order to communicate with the president "you need to make your points simple, even blunt".
In a scathing assessment of the White House, he said: "We don't really believe this administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction riven; less diplomatically clumsy and inept."