Danger for Boris Johnson, if he becomes PM, is being portrayed as governor of 51st state

In normal times, the breakdown in relations between Donald Trump and the British ambassador to the US could be readily finessed. Trump would vent his anger, and gain private British assurances that Sir Kim Darroch will be replaced by someone more palatable by the end of the year.

It would create an awkward interlude in the special relationship but every family has bust-ups.

But Trump does not seem to be in a mood to let this pass. As Darroch wrote in one of his leaked memos, the president radiates insecurity, and only one-syllable fawning keeps him on the rails.

Trump calls May foolish as diplomatic row escalates Read more

Theresa May, increasingly the target of Trump’s unfriendly fire, has so far clearly decided to stand up for Britain, the diplomatic profession and support Darroch by simply refusing to move him. She will have prevented a revolt in the civil service by doing so, and may feel relieved that the artifice of her friendship with the president is drawing to a close.

Yet Darroch is not scheduled to leave the post until the end of the year. Trump withdrew Darroch’s invitation to a state dinner for the Emir of Qatar, and clearly intends to go on in this mode. If he decides other US state institutions should blackball the ambassador, the president is entitled to issue the instruction. He can, if he so wishes, expel Darroch from the country. Emissaries are there at the pleasure of the country to which they are sent.

Play Video 0:33 Donald Trump: 'We're not big fans' of UK ambassador to US – video

Trump could even extend the embargo on contacts to all British diplomatic staff. In extremis, relations between the intelligence agencies could be curtailed. Trump anyway has a mixed view of British intelligence after one former agent, Christopher Steele, played a role in compiling the dossier on the president’s alleged collusion with the Russians during the 2016 election.

Trump, as he has shown innumerable times, is capable of anything, including a 180-degree turn in one tweet to the next.

In reality, May had little option but to stand by Darroch, given the insults hurled at her by the president and the country’s self-respect on the line.

It will be for the remaining grownups in the Trump administration, including the US ambassador to Britain, Woody Johnson, to try to limit the fallout by making sure Trump’s only target is Barroch and not the UK more broadly.

Liam Fox, who is close to the US administration and a loyal ally of May, was in Washington this week working overtime to achieve this, largely by trying to put Darroch’s memos in context and assuring the president that the ambassador’s selectively leaked views do not reflect UK government thinking.

The standoff could continue, but with time the president’s attention will stray on to another item on Fox News.

But May has only weeks left in office, and neither Boris Johnson nor Jeremy Hunt, if elected leader, will be able to avoid the hottest item in their in-tray when one of them becomes prime minister.

It should not be beyond the wit of some of the best diplomats in the land, and even Johnson, to assure Trump that his new bete noire – Darroch – is leaving imminently, and someone more palatable is on his, or her, way. Given the long lead time in which these major appointments are made, Johnson could plausibly announce a successor in September, as part of a wider shakeup, in time for the new person to take over in the late autumn.

But the bullying tone adopted by Trump does not make the choices easy for Johnson, a man that shares Trump’s vision of Brexit, and what it could mean for a reinvigorated special relationship.

Play Video 0:46 I have a good relationship with the White House, says Boris Johnson – video

The existing danger for Johnson is that if Trump lauds his elevation too much, he will find himself portrayed not as prime minister, but as the governor of the 51st state. If the consequence of Brexit is obeisance to Trump, that is not a good look, including among the patriotic classes that do not view national liberation in terms of swopping vassalage to Brussels for Washington.

On the other hand, if Johnson can show his friendship with the Trump family means he can persuade the president to de-escalate the crisis that would be a bonus.

In Tuesday night’s TV debate between Hunt and Johnson, both candidates are likely to be asked whether they would support someone from outside the diplomatic service taking up the Washington job, so by implication accepting there is some merit in Trump’s criticisms of the British civil service.

To appoint an outsider in the current context would be symbolic, and potentially perilous. Tom Fletcher, a former ambassador to Lebanon who is close to the Foreign Office, has been one of the most prominent former diplomats out defending impartial, professional, brilliant public servants. He warned: “We are in a fragile, unruly moment as a country – it is reckless to undermine such a fundamental and crucial foundation of our strength.”

It is striking how many ministers, whatever their frustrations, agree with this assessment. If the aim of the leak was to ensure that a free trading pro-American businessperson was given the ambassadorship in Washington, the whole risky enterprise may have backfired