Amber Rudd, the work and pensions secretary, who has hinted she could back Mr Johnson, refused – three times – to describe him as “an honest politician”.
“I would be very concerned about somebody who is too enthusiastic about no deal,” she warned, while ruling herself out of the race.
The charge was led by Mr Stewart, a rival for the top job, who dramatically ruled out serving under Mr Johnson because he wasn’t ready to carry out a “damaging, unnecessary and dishonest” no-deal Brexit.
Significantly, the international development secretary revealed the frontrunner had assured him a no deal was not his plan – only to later vow to take the UK out in October “deal or no deal”.
Speaking to The Independent, Ms James revealed she would back a Final Say referendum to block a no deal, adding: “I don’t trust Boris and I don’t think he is up to the job.”
The criticism came as:
* Labour predicted moderate Tory MPs would help topple an “extremist” new prime minister pursuing a no-deal Brexit – arguing there could be “a majority” in the Commons to stop it.
* Ms May was urged to use the dying days of her premiership to push through a law to prevent the UK crashing out – having become increasingly open about its dangers.
* Change UK’s leader, Heidi Allen, suggested up to 10 Conservative MPs would defect to her party if a hardline no-deal candidate became prime minister. And Chuka Umunna said the new party could form an electoral pact with the Lib Dems.
* Matt Hancock, the health secretary, became the fifth leadership contender – warning against both a no-deal Brexit or a general election that “would risk Corbyn by Christmas”
* Liam Fox, the international trade secretary, warned a crash-out was “increasingly likely” as the EU lost patience “with Britain’s constant delay”.
The warning bells sounded for pro-EU Tories after Mr Johnson, within hours of the prime minister’s resignation, ruled out a further Article 50 extension, insisting: “We will leave the EU on 31 October, deal or no deal.”
“I spoke to Boris, I suppose, about two weeks ago about this and I thought at the time he had assured me that he wouldn’t push for a no-deal Brexit,” Mr Stewart told the BBC.
“So, we had a conversation about 20, 25 minutes and I left the room reassured by him that he wouldn’t do this. But, it now seems that he is coming out for a no-deal Brexit.”
Later, in a barb clearly aimed at the former foreign secretary, Mr Stewart tweeted: “The star name will not always be the best choice.
“There may be times when Jiminy Cricket would make a better leader than Pinocchio.”
In recent days, the 60-strong One Nation group has hinted at a rapprochement with Mr Johnson, offering a huge boost to his chances of winning the leadership contest.
But Ms James said: “I’ve been disappointed to hear the mealy-mouthed words from people in the group who seem to be drawn to Boris because they think he will save their seats.
“If they think he is not fit to be prime minister, they should say that. I would not serve in a government led by him.”
John McDonnell, the shadow chancellor, said the UK was “faced with an extremist leader of the Conservative Party coming in, willing to take us over the edge of a no deal”.
“Faced with that situation, I think there may well be a majority in the House of Commons to bring about some form of public vote – and that could include a general election,” he predicted.
Mr McDonnell said Labour would be talking to nervous Tory backbenchers before staging an immediate no-confidence vote if a Brexit hardliner won.
The timing of the Tory leadership election, to conclude only days before Westminster’s summer break in late July, risks a constitutional crisis, some experts believe.
When challenged to prove they have a Commons majority, the winner must retain the support not only of anti-no-deal Tories, but also the Democratic Unionist Party which has propped up the May administration.
Following the defection of three Tories to the Independent Group, now Change UK, the working majority of the Conservative-DUP alliance is just six – meaning only four Tories would need to switch sides for a no-confidence vote to succeed.
If the new leader failed at that hurdle, and no alternative government is approved by the Commons within 14 days, there will be a general election.
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