Boris Johnson has insisted his lack of public support for Britain’s ambassador to the US did not tip Sir Kim Darroch into resigning his Washington post as he also denied he had made a bad situation worse over the imprisoned Nazanin Zagahri-Ratcliffe.

And his rival for the Conservative crown, Jeremy Hunt, has left open the prospect that Britain could still be in the EU by the start of 2020, refusing to guarantee he would, as Prime Minister, deliver Brexit by Christmas.

The two contenders to take over from Theresa May were each grilled for half an hour by the BBC’s political inquisitor Andrew Neil whose forensic questioning at times left both men looking decidedly uncomfortable.

Asked five times why he had not offered explicit support for the ambassador during ITV’s live head-to-head, Mr Johnson insisted: “I stood up completely for the principle that civil servants should be allowed to say what they want to their political masters without fear or favour.”

His lack of explicit support for Sir Kim led Sir Alan Duncan, the Foreign Office Minister, to accuse him of throwing the diplomat “under a bus”.

The former Foreign Secretary, who described Sir Kim as a “great public servant,” pointed out he had spoken to him after the TV debate, saying the ambassador had not seen it. “What he said was that somebody had relayed to him what I had said.”

Mr Johnson accepted Sir Kim had said that his relayed comments had “certainly…been a factor in his resignation” but he insisted his words during the live head-to-head had been “misrepresented”.

Earlier during his interview, Mr Hunt had said the ambassador had “been clear that that[Mr Johnson’s lack of support] was one of the factors and it's a great shame…We have to back our diplomats all over the world,” declared the Foreign Secretary.

When it was suggested Mr Johnson had been “craven” in the wake of Donald Trump’s public criticisms of Sir Kim, the Tory frontrunner replied: “Don’t be ridiculous…When it comes to sticking up for UK interests, whether it’s over climate change, over disputes with Iran over the Iran nuclear deal, we have been very, very forthright with the USA.”

Sir Kim resigned this week, saying his job had become “impossible” after the US President announced his administration would no longer deal with him. The ambassador’s leaked cables, in which he described the Trump White House as “inept” and “dysfunctional,” raised the President’s ire, leading to him branding Sir Kim a “pompous fool”.

HeraldScotland: Camley's Cartoon: Darroch resigns after Trump lashes out.Camley's Cartoon: Darroch resigns after Trump lashes out.

However, on Friday when he was speaking to reporters at the White House, Mr Trump said he had since been told Sir Kim had said "some very good things" about him. "Too bad…I wish the British ambassador well but they have got to stop their leaking problems there just like they have to stop them in our country."

On Ms Zaghari-Ratcliffe, Mr Johnson was repeatedly asked why he had wrongly said she was teaching journalism in Iran when she was on holiday. “You made a bad situation worse,” declared Mr Neil.

But the former Cabinet minister replied: “There’s no evidence for that. The reality is she is being detained at the behest of the Iranian Revolutionary Guard… I reject entirely the assertion that anything I said made things worse.”

In his interview, Mr Hunt insisted it was possible to get a revised deal with Brussels by the end of September but stressed that if it took "a few extra days" to get it through Parliament, he would delay Brexit beyond the October 31 deadline.

When asked if the UK would have left the EU by Christmas, the Foreign Secretary would only say: “I believe so.”

Pressed that he was unable to give a date, Mr Hunt replied: “Because I’m being honest with people. This is a negotiation. I don’t control the parliamentary timetable…A straight answer is not giving a date you can’t deliver.”