"We will get a deal by October 31st", Johnson told the BBC during highly-anticipated separate interviews with the leadership rivals, referring to the latest delayed deadline for Britain's departure from the bloc.
Mr Johnson's lack of explicit support for Mr Darroch during the ITV debate on Tuesday was widely seen to have been the final straw for the envoy following the leak of his diplomatic cables criticising Donald Trump's White House. Everyone knows that." "I think that every person who considers the evidence that companies have given, whether it's in the automotive sector, whether it's in the food sector, whether it's in aerospace, whether it's in industries up and down the country.
When asked whether the front-runner to replace her, Boris Johnson, understood the seriousness of the office of prime minister, May said she thought that both he and Jeremy Hunt understood the job's responsibility - although she did not endorse either candidate.
Tory leadership hopeful Jeremy Hunt has acknowledged he may "pay a political price" after saying he could not guarantee he would take Britain out of the European Union by Christmas.
But pressed on whether the United Kingdom would be out by Christmas, he said: "I'm not going to give you those commitments".
However, the Tory leadership frontrunner denied failing to give Mr Darroch his backing and said his words during a televised leadership debate had been "misrepresented".
But in contrast, his opponent Mr Johnson said it was "absolutely insane" to say that Britain is prepared to delay Brexit again. "There are other things - I think I probably actually should have done the TV debates".
Elsewhere, British Prime Minster Theresa May has said she will leave Number 10 with a mixture of pride and disappointment, as she admitted underestimating the "entrenched positions" in parliament over Brexit.
Mrs May said she had wrongly assumed MPs would be "eager to get Brexit over the line".
She said if the future relationship had been negotiated alongside there would not have been the same issue with the backstop.
The victor of the Conservative Occasion management rush - and subsequent top minister - will doubtless be introduced on 23 July. But she was sorry about having to leave when "there was more that I wanted to do".
She said she hoped her critics and supporters alike would indubitably feel that "in the whole lot I've accomplished, I've continually accomplished what I believed to be within the national curiosity".
Some 53% of those questioned, and who voted Conservative at the 2017 general election, say they would cast a vote for Johnson, against 29% who would choose Hunt. "But everybody in Parliament had a responsibility in how they voted on this issue".
It had been, she said, "incredibly frustrating" that MPs on either side of the Leave-Remain divide had "got so sort of entrenched that they just were not willing to make that compromise that would enable us to get the majority to get this through".