Boris Johnson faced a furious political backlash after he was accused of having thrown Britain’s top diplomat in America "under a bus" to further his own bid to become the next Prime Minister.

The blunt allegation from Sir Alan Duncan, the Foreign Office Minister, came after Sir Kim Darroch announced his resignation from his Washington post.

It was suggested the ambassador had concluded it had become “impossible” to continue in his role after Mr Johnson, the frontrunner to succeed Theresa May in No 10, failed to rally to his defence during Tuesday night’s TV head-to-head with Tory leadership rival Jeremy Hunt, but, rather, stressed the importance of UK-US special relationship.

Jeremy Corbyn’s spokesman branded Mr Johnson a “Trump patsy” and someone who was “not prepared to stand up to the US President and not prepared to stand up for Britain”.

ANALYSIS: Donald Trump's cold shoulder strategy spelled end for ambassador Sir Kim Darroch

Emily Thornberry, the Shadow Foreign Secretary, said the former London Mayor should "hang his head in shame" over his failure to support Sir Kim.

She argued: "The fact that Sir Kim has been bullied out of his job because of Donald Trump's tantrums and Boris Johnson's pathetic lick-spittle response, is something that shames our country."

Nicola Sturgeon said it was “shameful” Sir Kim had effectively been forced out for doing his job and noted: “Boris Johnson's failure last night to stand up for him - and stand up to the behaviour of Donald Trump - spoke volumes."

Her SNP colleague Stephen Gethins said the ambassador’s resignation was a "damning indictment of how unfit Boris Johnson is to hold high office and highlights his gross irresponsibility".

But the most outraged comment came from Mr Johnson’s former colleague at the Foreign Office. Sir Alan, who is a supporter of Mr Hunt, angrily claimed that it was "contemptible negligence" for the London MP not to have supported Sir Kim.

"He's basically thrown this fantastic diplomat under a bus to serve his own personal interests," declared the minister.

But Sir Michael Fallon, the former Defence Secretary, who is backing Mr Johnson for the party leadership, rallied to his defence, saying the frontrunner had made it "very clear that the relationship with the United States is what comes first".

Nigel Farage argued Sir Kim had made the “right decision," tweeting: “Time to put in a non-Remainer who wants a trade deal with America."

The former London Mayor himself expressed his regret at Sir Kim's departure, describing him as a "superb diplomat," who he had worked with for many years.

"It is not right that civil servants' careers and prospects should be dragged into the political agenda," he insisted.

READ MORE: Sir Kim Darroch quits as British Ambassador to US over Donald Trump leak row

The transatlantic drama began at the weekend when Sir Kim raised the ire of Mr Trump after his views about the US administration expressed in emails - that it was “inept” and dysfunctional” - were leaked to a Sunday newspaper.

The President took to Twitter to condemn Sir Kim as “wacky, branding him a "pompous fool". He also made clear his ministers and officials would “no longer deal with him”.

This resulted in:

*Sir Kim being “disinvited” to a White House dinner;

*absenting himself from a meeting with Ivanka Trump and

*Wilbur Ross, the US Commerce Secretary, abruptly cancelling a long-planned meeting with Liam Fox, the International Trade Secretary, because the ambassador was due to attend it.

In his letter of his resignation to the Foreign Office, Sir Kim said he wanted to put an end to the speculation surrounding his position, noting: “The current situation is making it impossible for me to carry out my role as I would like.

"Although my posting is not due to end until the end of this year, I believe in the current circumstances the responsible course is to allow the appointment of a new ambassador."

Just 30 minutes before the weekly Prime Minister’s Questions in the Commons, Sir Kim informed Mrs May of his decision.

At the start of PMQs, Mrs May related it to MPs, saying his departure was a "matter of great regret" while pointedly stressing the importance of backing public servants when they came under pressure.

“Sir Kim has given a lifetime of service to the United Kingdom and we owe him an enormous debt of gratitude,” she declared.

Earlier, Mr Hunt praised Sir Kim’s "unswerving devotion" to upholding UK interests, adding: “He brought dispassionate insight and directness to his reporting to ministers in London. I profoundly regret how this episode has led Sir Kim to decide to resign."

Meanwhile, Downing Street confirmed "initial discussions" had taken place with the police regarding the Whitehall investigation into the source of the leak.

"If there was concern about criminal activity, the police would become involved more formally at that point," a No 10 spokeswoman said.

Scotland Yard told The Herald there was, as yet, no formal police investigation as there was as yet “no referral of allegations in relation to the Official Secrets Act from the Government concerning the alleged leak”.

Last night, Downing St was on Twitterwatch. By 8pm, there was no response from Mr Trump to Sir Kim’s resignation.