A Tory leadership candidate says his competitors’ claims they could negotiate a new Brexit deal before 31 October are “misleading”.
The EU set the date for the UK’s exit after MPs rejected Theresa May’s deal.
Some candidates say they can agree a new plan by the deadline, but Rory Stewart said there was “not a hope”.
It follows Boris Johnson’s warning that the Conservatives face “potential extinction” if the UK doesn’t leave the EU by the end of October.
The former foreign secretary told a leadership hustings on Tuesday the party would “not be forgiven” if it failed, and said he was the candidate best placed to beat Labour and “put Nigel Farage back in his box”.
The comments came as the Eurosceptic European Research Group (ERG) of Tory MPs called on leadership contenders to abandon Mrs May’s Brexit deal – which was voted down in Parliament three times.
A paper published by the influential ERG said the next prime minister should sign up to the October deadline and step up preparations for a no-deal exit on World Trade Organisation terms.
International Development Secretary Mr Stewart told BBC Radio 5 Live’s Emma Barnett that anyone promising to renegotiate by October was effectively committing to leaving without a deal because it was impossible.
“Politicians need to stop pretending they are going to get a new deal from Brussels,” he said.
Mr Stewart and Mr Johnson are two of 11 candidates running to become the next leader of the Conservative Party, and the next UK prime minister.
On Tuesday, two pulled out of the leadership race as the party tightened the rules for the contest amid concerns about the size of the field.
Candidates will now need the support of eight MPs to take part in the race, and to secure of 5% of the vote in the first round, and 10% of the vote in the second round, to progress.
Charles Walker, the acting joint-chair of the Tory backbench 1922 Committee, said it was “not unreasonable for someone seeking to be leader of the party and prime minister to be able to muster” that level of support.
He told BBC Radio 4’s Today programme that he expected the Parliamentary side of the leadership contest – where MPs narrow the field down to two final candidates – to take no more than two weeks.
The winner will then be chosen by the wider membership of the Conservative Party.