Tory leadership rivals have clashed in a live BBC TV debate on whether the UK can leave the EU, no matter what, by the 31 October deadline.
Asked for a guarantee he would do this, Boris Johnson described the deadline as "eminently feasible".
Sajid Javid said it "focused minds", but Michael Gove and Jeremy Hunt said extra time might be needed.
Rory Stewart accused his colleagues of lacking realism - of "staring at the wall and saying 'believe in Britain'".
The five men vying to be Conservative Party leader - and the UK's next prime minister - were taking part in a live debate on BBC One on Tuesday night.
Former Brexit Secretary Dominic Raab was earlier eliminated in the second round of voting, when Conservative MPs held a secret ballot.
The third round of voting will take place later, between 15:00 and 17:00 BST. The result is expected at 18:00 BST and the MP with the lowest number of votes will be out of the race.
During Tuesday's hour-long debate, all five men ruled out calling a general election until Brexit was resolved.
But the encounter exposed divisions in their approaches to Brexit and whether they could accept the UK leaving the EU without an agreement.
The candidates, who faced questions from members of the public on issues ranging from climate change to Islamophobia, also disagreed over whether to prioritise tax cuts or increased spending on public services after the UK leaves the EU.
Mr Johnson, the frontrunner in the contest, was taking part in his first debate of the campaign after he skipped Sunday's Channel 4 encounter.
Rory Stewart, reflecting on the debate on BBC Radio 4's Today programme, said he had found it a "frustrating" format and wished he had pressed Mr Johnson further to explain how he would get a no-deal Brexit through without the consent of Parliament.
Looking ahead to the vote later, he said he had received a "couple of positive responses" to appeals for support from MPs who had previously given their backing to Dominic Raab.
Conservative MP Nadhim Zahawi, who has switched his support from Mr Raab to Mr Johnson, said he "certainly believed" the UK would leave the EU by 31 October under Mr Johnson.
Pressed on why Mr Johnson did not "guarantee" that on live TV, Mr Zahawi said: "If the other side don't believe you will leave on 31 October on WTO terms then they will not come forward with a deal."
He added that EU negotiators would take Mr Johnson "very seriously because they know he will prepare the country".
Jeremy Hunt said the EU needed to be able to trust the person they negotiate with. "I believe they would trust Jeremy Hunt, they would sit around the table, they would listen," he told the Today programme.
Asked if he was missing the charisma needed to win the leadership race, he said he had been underestimated all his life - in politics and in setting up his own successful business - and planned to surprise people.
How candidates' Brexit plans stacked up
The former foreign secretary said the British people were "fed up" with the current deadlock over Brexit and the Tories would pay a "really serious price" if this continued.
He warned of a "catastrophic loss of confidence in politics" if the latest Brexit deadline was not met. Asked if he could guarantee this, he replied that "October 31 is eminently feasible".
"If we allow 31 October to come and go as we let March come and go, I think the public would look on us with increasing mystification," Mr Johnson said.
He also suggested there was no issue with continuing free trade after Brexit - citing something called Article 24 of GATT - but as BBC Reality Check points out, that relies on the UK and EU both signing up and in the event of no deal, that will not happen.
Mr Javid, who came fifth in Tuesday's second round of voting, said a deadline was needed to "focus minds" in both the EU and the UK.
"We have to learn from our mistakes," he said. "One of the mistakes we have made is having a flexible deadline."
He suggested the route to getting a Brexit deal through Parliament was by re-presenting Theresa May's Withdrawal Agreement but without the controversial Irish backstop.
Mr Gove said an "arbitrary" deadline was counter-productive and if he was prime minister he would be prepared to delay Brexit by a matter of days to finalise a deal.
"You sometimes have extra time in football matches in order to slot home the winner."
Mr Hunt said he would back a no-deal exit as a "last resort" but if the UK was close to finalising a deal with the EU he would extend the talks to prevent the disruption a no-deal exit would cause to business.
Both Mr Hunt and Mr Javid suggested new technology could potentially solve the intractable Irish border issue, but the EU has said there is currently none in use anywhere in the world that can keep it as open after Brexit as it is now.
Mr Stewart said he would not allow a "damaging and unnecessary" no-deal exit and his rivals could not explain how they could possibly do this "against the consent of Parliament".
He suggested he was seeking the most realistic "door" out of the EU while "everyone else was staring at the wall and saying believe in Britain".
What Europe made of the debate
EU politicians across the continent were dipping in and out of the debate. The comments I've heard so far off-the-record have not been particularly complimentary.
The EU simply thinks that most of those leadership candidates are not being realistic.
EU leaders are preparing a united, determined front when it comes to the idea of renegotiating the Brexit deal, and the answer is no.
Even if, come the autumn, the EU were to be tempted to reopen some of those questions such as the Irish backstop, those conversations could never be finished by 31 October - the date by which most of those leadership candidates want to leave the EU.
That's why this evening the EU thinks the idea of a no-deal Brexit is becoming increasingly likely.
Tax plans also a dividing line
Moving beyond Brexit, the candidates clashed over their economic plans and whether to prioritise higher spending on public services or tax cuts.
There were sharp exchanges between Mr Stewart and other contenders, Michael Gove accusing him of having "no plan" for how to run the economy or public services.
"Bringing people together is not enough," he told Mr Stewart, who has emerged as the surprise contender in the race.
However, after the debate Mr Stewart admitted his performance had been lacklustre, adding he did not think the "strange" format worked for him.
Mr Gove also took aim at Mr Johnson's plans to give a tax cut to those earning more than £50,000 a year, saying the focus should be on "helping the poorest in society".
Helping middle-earners was "sensible", Mr Johnson responded.
Mr Stewart said promising tax cuts was "wrong" given the uncertainty around Brexit.
He called for a "revolution" in care for the elderly, calling current provision a "disgrace".
Mr Hunt, who was health secretary for six years, also called for increased investment, suggesting cuts to care budgets under the current government had gone "too far".
Mr Johnson reiterated that it was his "ambition" to cut taxes for higher earners, but as political correspondent Chris Mason pointed out, that seemed less committal than a "promise" to do it.
Plenty expected lots of ganging up on Boris Johnson tonight from his rivals.
But what was striking - after the Stewart surge in votes between rounds one and two in Parliament - is there was at least as much, if not more, ganging up on Rory Stewart.
Look at the numbers.
There were just 13 votes separating Jeremy Hunt, who finished second, and Sajid Javid, who finished fifth in the leadership ballot.
In other words, all of them, other than Boris Johnson, are potentially vulnerable to relatively small shifts in votes.
And Mr Stewart, in the build up to tonight, had the momentum.
Meanwhile, speaking to Newsnight after the debate, Mr Gove said he believed he had won the debate because of what he said were his "detailed answers" and "clear plan" on Brexit.
Of his rivals, he said there were "some other great people" but added: "They will all be fantastic members of my team."
Challenge over Islamophobia
Amid claims that the Conservatives have failed to tackle Islamophobia in the party, the candidates were pressed by an imam to accept that "words have consequences".
Mr Johnson said he apologised if anything he had written, during 20 to 30 years as a journalist, or had said during his political career had caused offence.
But he defended his conduct as foreign secretary in relation to the case of Nazanin Zaghari-Ratcliffe, who remains in jail in Iran on spying charges.
He suggested his claim in 2017 that the dual British-Iranian national was actually working as a journalist in the country - which the Iranian authorities cited as a reason to increase her sentence - "did not make any difference".
"If you point the finger at the UK, all you are doing is exculpating those who are truly responsible," he said.
Mr Javid challenged the other candidates to agree to an external inquiry into Islamophobia in the Tory Party - which they all appeared to do.
Referring to Donald Trump's string of attacks on London's Muslim mayor Sadiq Khan, he said politicians should be "brave enough" to call out Islamophobia wherever it came from.