Boris Johnson has been given a fresh warning of the challenges he will face if he becomes prime minister as Philip Hammond promised to do “everything in my power” to block a no-deal Brexit.
The Chancellor left open the possibility of voting to bring down a Conservative government led by Mr Johnson if the UK was on course to crash out of the European Union without a deal on October 31.
Mr Johnson has again ruled out seeking an extension to the deadline, insisting that the UK would leave the EU on Halloween “come what may”.
But Mr Hammond said it would be impossible to renegotiate a deal with Brussels before that deadline.
In an interview with European newspapers, he said: “I will take steps to avoid an exit without agreement apart from an explicit parliamentary approval.
“There should be a new and sincere attempt to reach a consensus.”
Hinting at the prospect of another referendum, he told Le Monde and Suddeutsche Zeitung: “If we do not find a solution with the members, we may have to ask the British to give their opinion again, in one form or another.”
Asked again to rule out supporting a motion of no confidence, the Chancellor said: “I do not exclude anything for the moment.”
The scale of the task facing Mr Johnson if, as expected, he defeats Jeremy Hunt in the Tory leadership battle, was laid bare in the Commons on Thursday, as Cabinet ministers who expect to return to the backbenches put down a marker about their willingness to cause trouble.
MPs voted by a majority of 41 to back a measure aimed at preventing Mr Johnson suspending Parliament in order to force through a no-deal Brexit, with 17 Tories rebelling and dozens more abstaining.
The Chancellor, Justice Secretary David Gauke, Business Secretary Greg Clark and International Development Secretary Rory Stewart did not vote, all are expected to be on the backbenches under Mr Johnson and could be a thorn in his side.
In a signal of his intentions as a backbencher, the Chancellor said: “I will remain a member of the House of Commons.
“I will do everything in my power from my position to make sure that parliament blocks a Brexit without agreement.”
Another frontbencher, Health Minister Stephen Hammond, signalled on Friday that he too would be willing to vote down a Johnson administration in a no-confidence vote if it pursued a no-deal exit.
Asked whether he was prepared to press the “nuclear button” with such a vote, he repeatedly cited putting the national interest before personal ambitions.
“I hope we never get there, but I think a lot of people were taught that you must put the interest of the country before yourself,” the Wimbledon MP told BBC Radio 4’s Today programme.
“I don’t think we will get there actually, and I’m pretty certain, as a Conservative, that I would be very, very, very cautious about ever doing that.”
He added: “I think it’s really important that, at this historic stage in this country’s lifetime in modern politics, that politicians put aside any of their own personal ambitions or views and actually make sure they do the right thing as they see it for the country.”
Tory Eurosceptic Jacob Rees-Mogg, an ally of Mr Johnson, poured cold water on the significance of the Commons vote aimed at preventing the new prime minister suspending Parliament.
“Something that was never going to happen is now slightly harder to do,” he told Today.
The Tory MP also suggested the likelihood of a successful no-confidence vote was slim, accusing MPs of trying to “bluff” their way to preventing Brexit.
“It’s quite difficult unless they have a vote of no confidence and they don’t want to do that because they would lose their seats because any Conservatives who voted against the Government on a vote of no confidence would be automatically deprived of the whip,” he said.