Boris Johnson and Jeremy Hunt escalated the blue-on-blue warfare in Tuesday night’s televised debate, trading blows on which potential prime minister could see through a no-deal Brexit, as Hunt repeatedly emphasised his rival’s refusal to answer questions.

The bitter back-and-forth, which will raise questions over whether Hunt could serve in a Johnson government after the aspersions he cast on his rival, saw the foreign secretary accuse Johnson of putting personal ambition above the welfare of the country.

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In the hour-long ITV broadcast, Johnson swerved questions repeatedly on whether he would resign if he failed to deliver Brexit by 31 October and declined to condemn Donald Trump for his attacks on the British ambassador.

Johnson also gave equivocal answers on issues including HS2, Heathrow’s third runway, and abortion and LGBT rights in Northern Ireland.

Johnson said that committing to resign if he fell short of his Brexit promise would play into the hands of the EU, and he also refused to rule out proroguing parliament to deliver no deal, calling it “absolutely bizarre” to take the option off the table.

Hunt said such a scenario could potentially lead to civil unrest. “When parliament has been shut down against its will, we’ve actually had a civil war … my answer to that is no,” he said.

In a number of pointed attacks in the first head-to-head debate of the Conservative leadership race, Hunt suggested the EU would not negotiate with someone who had “gratuitously insulted them” and repeatedly challenged Johnson to resign if he did not deliver Brexit by 31 October.

In turn, Johnson repeatedly called Hunt “a defeatist”, hinted the country had had enough of a “managerial style” and called his rival “a stickler for detail … and you tend to go on about it”.

Johnson, widely tipped as the frontrunner in the race, at one point suggested he was uncomfortable with the tone of the debate. “This is one of the reasons these blue-on-blue debates are so embarrassing,’” he said.

Hunt repeatedly stressed Johnson’s unwillingness to commit on detail. “You have not answered any of my questions. Not one,” he said. “Because Boris never answers the question, we have no idea what a Boris Johnson premiership would be like.”

The roughest exchanges came as the two men clashed over the prospect of leaving the EU on 31 October, deal or no deal. Hunt repeatedly challenged Johnson to say if he would resign if he could not deliver an exit by Halloween,

“Delay does not deliver a deal,” Johnson said. “I think it’s very important not to envisage any circumstances where we fail to come out 31 October. I don’t want to hold out any prospect to the EU that they might encourage my resignation.”

Hunt suggested that it was “Boris in No 10” that was the priority for Johnson.

“I asked Boris a straight question and he said very clearly before that it was leaving on 31 October do or die,” he said. “And I think it’s do or die for the country – but not a prime minister who would put his own neck on the line – and that is not leadership.”

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In turn, Johnson repeatedly mocked his opponent for failing to give a firm exit date after Hunt said he believed a hard exit date increased the risk of a general election. “When is the deadline? Christmas? Is Christmas any good?” he asked.

Johnson said the warnings that parliament would stop no deal were overstated. “It is totally defeatist not to set a hard deadline. Parliament has just resisted the temptation to vote down no deal. An outbreak of realism is taking place across our politics,” he said.

Other areas where the two men clashed included Johnson’s proposed tax cuts for workers earning between £50,000 and £80,000, his first pledge of the campaign.

Though Johnson insisted those would be “part of the package” which would include measures for those on low incomes, Hunt said it was tarnishing the image of the party.

“I have spent my life trying to convince people we are not the party of the rich, we are the party of everyone, and if the very first tax cut you announce is a tax cut for the rich, that is the wrong signal to send about our party,” he said.

There was no common ground even in the final moments of the ITV debate – when each candidate was challenged to find a point of admiration for the other. Johnson initially floundered and then highlighted Hunt’s previous support for remain. “I greatly admire his ability to change his mind and campaign for Brexit now; it is an important attribute,” he said.

Hunt’s response was even more pointed. “I really admire his ability to answer the question, to put a smile on your face and you forget the question. It is a great quality in a politician, maybe not in a prime minister,” he said.

Sources close to Johnson, who has avoided all other opportunities to take part in a head-to-head with Hunt, suggested they had been forewarned that Hunt intended the debate to be aggressive and decided their candidate had to respond. “That’s why these debates are a problem,” the source said.

The ITV debate – the only head-to-head in the campaign – came after ballot papers had already been distributed to Tory party members, raising questions as to whether anything in the debate would influence how members will vote.

Sources close to Hunt said they did not believe the debate would be an irrelevance and that anecdotal evidence from local associations suggested many were still undecided.

In the Johnson camp, there was little concern as to whether their candidate had swerved too many questions. “The whole frame of this contest is about Brexit and when we are going to leave and that is the one thing where Hunt is refusing to answer. Fundamentally that is the only thing that matters, everything else is just bluster and noise.”

A source close to Hunt said: “Boris Johnson, putting himself forward for proper scrutiny in a debate format for the first time since the contest entered the membership phase, exposed a plan for Brexit and for Britain devoid of detail and littered with factual errors and [showed us] a man who cannot answer basic questions about how he would lead the country.”