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The Conservatives face "potential extinction" if they do not get Brexit done, Boris Johnson has warned.

The former foreign secretary told a leadership hustings the party will "not be forgiven" if it does not take the UK out of the EU by 31 October as planned.

He said he was best placed to beat Labour and "put Nigel Farage back in his box" but reportedly ruled out a snap general election if he becomes PM.

Mr Johnson is one of 11 candidates vying to succeed Theresa May as leader.

His comments came as the Eurosceptic European Research Group (ERG) called on leadership contenders to abandon Mrs May's Brexit deal.

A paper published by the influential ERG said the next prime minister should sign up to the 31 October deadline and step up preparations for a no-deal exit on World Trade Organisation terms.

On Tuesday, two candidates 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 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.

Mr Johnson, Home Secretary Sajid Javid, International Development Secretary Rory Stewart and ex-Commons leader Andrea Leadsom addressed the One Nation Conservative Caucus of Tory MPs in Parliament at the first hustings of the campaign.

The group, which opposes a no-deal Brexit, has invited all the candidates to appear before them to make their case for the top job.

Mr Johnson, regarded as the frontrunner in the contest, said the party was facing an "existential crisis" following its drubbing in last month's European elections.

"We will not be forgiven if we do not deliver Brexit on October 31," he said.

"We need to realise the depth of the problems we face. Unless we get on and do this thing, we will be punished for a very long time. There is a very real choice between getting Brexit done and the potential extinction of this great party."

His warning was echoed by fellow leadership hopeful Ms Leadsom, who said a delay to the UK's exit from the EU could "spell the end" for the party.

'Managed exit'

Mr Johnson suggested the solution to the current deadlock would be to replace the Irish backstop - the controversial insurance policy designed to maintain an open border - with "alternative arrangements" so as to facilitate a "managed exit" from the EU.

The EU has said the backstop must remain in the withdrawal agreement and has resisted calls from Tories and the DUP for a time limit, or exit date, to be put on it.

The Tory leadership hustings hosted by the One Nation Caucus of Conservative MPs began with reporters, me included, starting to loiter outside.

Between us and the action inside, there were two heavy wooden doors and a pretty thick wall - and some parliamentary security staff not particularly keen on us leaning too obviously against either the doors or the wall.

A rather forthright conversation then began between us lot in the press pack and Conservative Party officials about why we weren't allowed in - given those in the room were discussing who should be our prime minister by the end of next month.

Read more from Chris here

Mr Johnson, who has insisted the UK must leave by the end of October with or without a deal, said a no-deal exit would cause "some disruption".

But he warned that demands for another referendum on the UK's future in Europe would grow if the UK was forced to seek another extension from the EU.

He insisted: "I believe I am best placed to lift this party, beat Jeremy Corbyn and excite people about conservatism and conservative values."

Home Secretary Sajid Javid told the meeting that if his party "don't look like change, voters will go for change in the form of (Jeremy) Corbyn".

He stressed that the party should not seek to emulate Nigel Farage's Brexit Party, insisting "we will not beat the Brexit Party by becoming the Brexit Party".

In a reference to Benjamin Disraeli's "one nation" political philosophy, he added: "One nation is a term that was coined by a prime minister who was a bit of an outsider. Pick a prime minister who is also a bit of an outsider."

International Development Secretary Rory Stewart told the event: "I began this race believing I should be a truth-teller on principle - ironically I have discovered that it is very popular - and the only way to avoid an election and win the next one is by being straight with people.

"No more unicorns, no more red lines, no more promises we can't deliver. That's how we get Brexit done, defeat Corbyn and unify the country."

He also told MPs he would not sign a confidence and supply deal with the DUP in exchange for funding, as Mrs May did.

Who will replace Theresa May?

The winner of the contest to lead the Conservative Party will become the next prime minister.

Speaking to BBC Radio 4's Today programme, Defence Secretary Penny Mordaunt would not be drawn on whether she planned to enter the leadership race.

But she claimed that nobody wanted a no-deal Brexit, and the EU "understand they have to move on some things".

She told the programme: "I stayed in cabinet and fought to try and get a deal and to try and build a consensus, both in my party and but also in Parliament.

"What we have learnt though is if you are trying to get that objective, you can't take no deal off the table."

Ms Mordaunt added: "In recent days, in my discussions that I have had with people on the EU side of the negotiating table, I am really optimistic and I think they understand they have to move on some things.

"I voted to Leave and I still remain optimistic that we can get a good deal for the UK."