President Donald Trump has intensified the crisis in relations with the UK, its closest ally with which it supposedly enjoys a “special relationship”, by tweeting “we will no longer deal” with Kim Darroch, the British ambassador in Washington.

In his latest tweet, Trump also laid into the outgoing British Prime Minister Theresa May, who retaliated by declaring the British ambassador had her “full support”.

Clearly smarting from Darroch’s depiction of his administration as “dysfunctional” and “clumsy and inept” —the ambassador’s confidential reports to London have been leaked to the Mail on Sunday — Trump said in his latest tirade: “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 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!”

The British government’s position is that the leak of the ambassador’s messages is regretted, and while it does not share his view of Trump’s administration, Darroch was merely doing his job.

His time in Washington is coming to an end but to fast track his departure would send the wrong signal to British diplomats around the world who are expected to report candidly back to London.

In his analysis of “Ambassadorgate”, the BBC’s diplomatic correspondent James Landale pointed out: “By saying he won’t deal with Kim Darroch any more, Donald Trump is apparently all but declaring the ambassador to be persona non grata. That is the formal legal process by which a host government expels a foreign diplomat.

“The key question now is what the President means by the word ‘deal’. If the royal ‘we’ used by Trump means that his entire administration will no longer deal with Darroch or any of his staff then the British government may have to decide to fast track the retirement of their man in Washington.”

“Darroch, who is an honourable man and was stepping down anyway in a few months, may decide to resign. If, however, Trump merely means he won’t deal personally with Darroch then the ambassador may stay on until the new Prime Minister can make his own appointment.

“This all presents the British government with an awkward dilemma —– to buckle under US pressure and bring Sir Kim home, risking accusations of abject weakness, or to stand firm and defend their ambassador for doing his job and telling the truth as he sees it, risking even further damage to the UK/US relationship.”The Brexit leader, Nigel Farage, who is Trump’s principal cheerleader in Britain, and a few other British politicians have called for the ambassador to be sacked but that is still a minority opinion.

Giving her assessment of the crisis, the BBC’s political editor Laura Kuenssberg, who has no doubt been briefed by British government sources, said: “Downing Street’s response is a classically formal ‘thanks, but no thanks’.”

A stiff brush-off in riposte to the US president’s digital tirade, which was extraordinary even by his standards.

“With the current Prime Minister almost out of the door, and the UK ambassador in Washington leaving too, the remarks are unlikely to change much directly, and this allows Number 10 to try to shrug off the criticism.

“Less officially, though, there is real frustration. One senior Tory warned that ‘we cannot bow down to this form of lunacy’ where the leader of another country tries to use online swagger to seek revenge on one of the UK’s diplomats — not least from one of our most important allies.”

The two Tory leadership contenders — Boris Johnson, a former foreign secretary, and James Hunt, his successor and the current incumbent — are going to be put on the spot and asked to declare who they support: Britain or America, Trump or the British ambassador?

A former British ambassador in Washington, Sir told BBC Radio 4’s Today programme: “It just shows President Trump’s sensitivity. His insecurity, which Sir Kim himself bore witness to.”

Referring to whoever leaked the diplomatic cables, now the subject of an official foreign office inquiry, Meyer added: “Here there is a possible range of villains who come into the frame. But, it was clearly somebody who set out, deliberately, to sabotage Sir Kim’s ambassadorship, to make his position untenable, and to have him replaced by somebody more congenial to the leaker.”

There have been exchanges in the Commons where Tom Tugendhat, chairman of the 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 was found, telling the Commons: “If evidence of criminality is found, then yes, the police could be involved.”

For Labour, shadow foreign secretary Emily Thornberry said the ambassador had been “betrayed” and “hung out to dry even though his only crime was to tell the truth” while Conservative former defence secretary Sir Michael Fallon said the ambassador had the confidence of parliament.

But this pledge of solidarity was undermined moments later as Sir Bill Cash damned Darroch: “These toxic and unjustified attacks on the President of the United States and his administration are regarded by many people as completely unjustified.

“Surely it is not his so-called frankness which should be the issue but his lack of judgment which disqualifies him from his post.”

Duncan said he considered Cash’s comments “deeply unworthy” and said of the ambassador: “Nothing in his reporting from the embassy could ever be construed as an attack on the President of the United States.”

Unless Trump changes his tune, this is a crisis like no other. Perhaps the wisest words came from the former Tory leader and foreign secretary, William Hague, now a peer, who said in an article in The Daily Telegraph: “Britain has one of the largest diplomatic networks in the world, and each working day most of our 240 embassies and consulates around the world send in a report. Foreign Secretaries vary greatly in how much use they make of these ‘diptels’ – diplomatic telegrams – but since I love reading, I used to go through the whole lot every night.

“If everybody could read the foot-high pile of information received each day they would develop a pretty favourable impression of our diplomats. Usually well written, the reports are always candid, sometimes amusing, often prescient and add up to a daily crash course in world affairs. A regular reader of them should very quickly become one of the best-informed people on the planet.

“Sir Kim Darroch, Her Majesty’s Ambassador in Washington, was therefore doing his job in the leaked and controversial reports the whole world now knows he has despatched to Whitehall about the Trump administration. His observations that the current White House has been ‘dysfunctional, unpredictable, faction-riven and diplomatically clumsy’ would, of course, be commonplace for anyone who troubled themselves to read a newspaper. But his observation that the President never intended to go ahead with aborted air strikes on Iran is an important insight. British ministers need to know that, if it is what their man in Washington thinks.

“If such reports ever had to be composed with an eye on avoiding giving offence to the foreign government or country in question they would be utterly useless.

“The best response to these leaks, however, is not to go into a funk of embarrassment, self-doubt or talk of removing our Ambassador before the end of his term. Indeed, the idea that somebody who tells the truth as he sees it should be asked to leave his post prematurely should be completely unacceptable in any part of government.

“After Brexit we are going to need more than ever that strong and professional network of the most able diplomats we can recruit and train. If, however, we stop asking the most experienced among them for the unvarnished truth we might as well not bother.”