Jeremy Corbyn could become prime minister without a general election next month if Conservatives elect a leader who cannot command a majority of MPs in the Commons, one of the contenders to succeed Theresa May has warned.

Parliamentary rules state that after a prime minister loses a confidence vote in the Commons, an alternative PM has 14 days to show that they can secure a majority of MPs before a general election is triggered.

Leadership candidate Mark Harper said it was “perfectly possible” that Mr Corbyn would be invited to try to form an administration, and would then go into the election as the incumbent prime minister.

Mr Harper said that in these circumstances, it was possible that those Tory MPs who are vehemently opposed to leaving the EU without an agreement would accept as a last resort to avoid a Corbyn premiership.

But, in an apparent swipe at front-runner Boris Johnson, he said they would only do so if they felt the government had “busted a gut” to get a deal, and not if it had been “blundering around like a bull in a china shop”.

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Mr Harper said it was “not credible” for candidates such as Mr Johnson to suggest that they could renegotiate Theresa May’s withdrawal agreement by 31 October or to persuade MPs to accept it unchanged by that time.

Declaring that it was “not going to be possible to leave by 31 October”, he said he would be ready to extend negotiations as late as May 2020, when local elections are due in the UK .

His preference was for EU withdrawal with an agreement, but no-deal had to remain on the table and he was"prepared and comfortable" to go through with it if necessary.

Mr Harper, who was not appointed to a ministerial post under Theresa May, said he could offer a “fresh” approach to Brexit.

Everyone else in this race has at some point in the last three years been sat around the cabinet table and has participated in the decisions that have led to us not leaving the EU three years after the referendum,” he said.

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“Every single one of them has participated in the fundamental misjudgments that have got us to where we are."

As constitutional reform minister, Mr Harper was one of the authors of the Fixed-Term Parliaments Act of 2011 which set out the 14-day window to find an alternative government following a confidence vote. He said it had been decided not to set out “prescriptive” rules on who should be invited to try to form an administration.

Tory MPs contemplating a no-confidence vote to remove a no-deal PM “wouldn’t know what was going to happen”, he said.

“It would be perfectly possible that if people voted against a confidence motion they could end up with Jeremy Corbyn becoming prime minister without a general election,” he said.

“We would then go into a general election with Jeremy Corbyn already being prime minister and the Labour Party in government.”

Given the threat of a Corbyn government and given the fact that voters had punished Tories at the European elections for failing to deliver Brexit, he said he believed that a majority of MPs would be ready to vote for no-deal “if they are faced with a government that’s busted a gut and done everything humanly possible to get a deal but just hasn’t been able to get one that can get through parliament”.