Theresa May said to be considering controversial step of appointing new envoy

Boris Johnson is under pressure over his role in Sir Kim Darroch’s resignation as British ambassador to Washington, with critics accusing the likely next prime minister of throwing the envoy “under the bus”.

In a shock move which prompted the senior civil servant at the Foreign Office to call an all-staff meeting to reassure “shaken” diplomats, Darroch announced on Wednesday he could no longer continue in his role following a leak of official cables in which he criticised Donald Trump.

The Guardian understands that he concluded he could not go on after he watched Tuesday’s Conservative leadership TV debate, where Johnson repeatedly dodged questions about whether he would sack the ambassador if he became PM.

Downing Street is so uncomfortable with the outcome that Theresa May is understood to be considering appointing a new ambassador in her last week as prime minister. That would upend protocol amid concerns Johnson could seek to make a controversial political appointment in the hope of pleasing Trump.

A broad political consensus has emerged that Johnson’s refusal to back Darroch had made the diplomat’s position untenable. Senior Conservative MPs joined the direct criticism of the leadership contender, describing his conduct as “unedifying” and “contemptible”. Tom Tugendhat, the chair of the foreign affairs select committee, asked: “If you do not support those you put into very difficult positions, what do you think is going to happen?”

In language that will be viewed as an implicit rebuke of Johnson, Theresa May told MPs: “I hope the house will reflect on the importance of defending our values and principles, particularly when they are under pressure.”

It is understood that Johnson, who worked with Darroch as foreign secretary, later spoke to the departing ambassador to express his “regret” at the fact he had resigned. Asked why he had not been more supportive of Darroch he insisted he had backed him and said it was “wrong to drag civil servants into the public arena”.

Play Video 1:10 Boris Johnson calls Kim Darroch a 'superb diplomat' – video

Meanwhile, a renewed focus also fell on the identity of the leaker, with Foreign Office permanent under-secretary Simon McDonald telling MPs: “We will pursue the culprit with all the means at our disposal. The leaker is guilty of the worst breach of trust in our service in my career.”

Asked whether the entire establishment would be expected to support Darroch in such an episode, he said that the Foreign Office had noted with gratitude the support given to Darroch by the prime minister and the foreign secretary. He made no mention of Johnson.

While the ambassador’s decision to resign brought the immediate crisis to an end, it sparked a new round of recrimination and investigation in the UK and fears that the transatlantic diplomatic calculus could have permanently shifted, as:

• No 10 said that discussions had begun with police over an investigation into the source of the leak.

• Amid fears of more leaks, the foreign affairs select committee heard that inquiries would focus on a set of highly sensitive emails sent to as few as five or 10 people.

• President Trump did not immediately add to his previous comments on the situation, but one senior US official said that the move was “probably the right course”.

• US-UK relations appeared further strained by the developments as McDonald warned: “There must be consequences.”

Darroch will stay in the role until a new ambassador is appointed – which could be imminently if May makes the controversial decision to pre-empt her successor. A government source said “discussions are live” and that she was expected to come to a decision within 24 hours.

As well as concerns over the nature of a Johnson appointment, there are worries that such a crucial vacancy should not be left unfilled.

McDonald told the foreign affairs select committee that he had been forced to call the all-staff meeting to reassure concerned diplomats and he feared that further illegal leaks could be in the offing.

“People are shaken by what has happened and there is a reason why I have asked to see all my colleagues,” McDonald said. “The basis on which we have worked all our careers suddenly feels challenged.” He described the episode as a personal tragedy for Darroch.

Play Video 1:29 Foreign Office chief says he fears more diplomatic memo leaks are coming – video

Sir Alan Duncan, a Foreign Office minister, attacked Johnson’s role in the affair.

“For someone who wants to lead, let alone unite, the country, that was contemptible negligence on his part,” Duncan told the BBC. “He has basically thrown this fantastic diplomat under the bus to serve his own personal interests.”

The Conservative MP Sir Patrick McLoughlin agreed, saying: “It is unedifying to see someone who wants to be prime minister failing to stand up for hard-working civil servants, who have done nothing wrong, under attack from foreign governments. Leadership involves standing up for your team.”

And the former Foreign Office minister Alistair Burt said: “Anyone, I think, would have seen last night’s events and seen a potential prime minister letting someone go very publicly.”

Replying to Darroch’s letter of resignation, in which he said that speculation was “making it impossible for me to carry out my role as I would like”, McDonald said Darroch had been the target of “a malicious leak” and expressed gratitude for his work. “You are the best of us,” he wrote.

Kim Darroch has resigned. Now Britain risks becoming a vassal of the US | Martin Kettle Read more

May told the House of Commons she had told Darroch it was “a matter of great regret that he has felt it necessary to leave his position”. Jeremy Corbyn called the treatment of Darroch “beyond unfair and wrong” and said he had given “honourable and good service”.

The criticism of Johnson was based on his choice of language in Tuesday night’s Conservative leadership debate, in which he refused to give Darroch his support, even as his leadership rival, the foreign secretary, Jeremy Hunt, said he would expect the ambassador to stay in post until his planned retirement.

Hunt said in a statement that he was “deeply saddened” by the outcome, adding: “I am outraged that a selection of his reports should have been leaked.”

At the foreign affairs committee hearing, McDonald said that there had been a great deal of soul-searching about the leak and added that he knew of no precedent where a friendly power had refused to cooperate with a serving British ambassador.

Play Video 0:54 'It is a matter of great regret,' says Theresa May as Darroch resigns – video

Asked what the impact would be for transatlantic relations he said: “It is too soon to have a complete or authoritative judgment. Nothing like this has ever happened. There must be consequences.” He later insisted the special relationship was so deep and so wide that it could survive any individual squall.

There was no immediate comment from Trump, but the vice-president, Mike Pence’s chief of staff, Marc Short, said that the move was “probably the right course”.

A state department spokeswoman said: “The United States and the United Kingdom share a bond that is bigger than any individual, and we look forward to continuing that partnership. We remain committed to the US-UK special relationship and our shared global agenda.”