Electing Boris Johnson leader on a no-deal Brexit ticket would risk a permanent split in the Conservative Party, a former minister has warned.

The former foreign secretary has established himself as firm favourite to succeed Theresa May as prime minister in a contest many in Westminster expect to be triggered within weeks.

It comes as the prime minister prepares to deliver a speech on Brexit this week, in what a government source billed as a "bold offer" to MPs in a last-ditch attempt to build support for her beleaguered Brexit bill in the Commons.

A poll conducted shortly after he confirmed he would stand on Thursday put him on 39 per cent support among party members, well ahead of his nearest rival Dominic Raab on 13 per cent.

But he must first win his way through an MPs’ vote onto the shortlist of two presented to the membership in the country.

Foreign secretary Jeremy Hunt is currently thought to be leading in the race to sign up MP supporters, with Mr Johnson, Mr Raab and Michael Gove grouped together some way behind, and a wide range of other potential candidates yet to make real inroads.

Who could succeed Theresa May as Conservative leader? Show all 9 1 /9 Who could succeed Theresa May as Conservative leader? Who could succeed Theresa May as Conservative leader? Boris Johnson Former foreign secretary Boris Johnson has long been hopeful, he previously stood in the leadership contest that followed the Brexit vote and has at many times since been thought to be maneuvering himself towards the goal. He remains a darling of the party's right wing, particularly those in the ERG, and is the most popular choice among Tory voters but his leadership bid would be fiercely opposed by many MPs PA Who could succeed Theresa May as Conservative leader? Michael Gove Environment secretary Michael Gove is another member who has long wanted to be leader. He has lately been known for rousing his party in the commons, his recent speeches on the Brexit deal and Labour's no confidence motion have overshadowed the Prime Minister's. He has been loyal to the Prime Minister, partly to shed his reputation as a backstabber who abandoned Boris Johnson to stand against him in the 2016 leadership election Getty Who could succeed Theresa May as Conservative leader? Dominic Raab Former Brexit secretary Dominic Raab has emerged as a favourite to be the Brexiteer candidate in a contest to succeed to Ms May. He displayed a grip on detail in his role as Brexit secretary. When asked recently if he would like to become prime minister he replied "never say never" Getty Who could succeed Theresa May as Conservative leader? Rory Stewart International development secretary Rory Stewart is pitching himself as the sensible candidate, promising to rule out both a second referendum and a no-deal Brexit. He was only recently promoted to the cabinet, previously serving as prisons minister, where he caught headlines with a pledge to resign if he could not reduce levels of violence within a year PA Who could succeed Theresa May as Conservative leader? Esther McVey The former work and pensions secretary announced that she will be standing for the leadership when May leaves. McVey is the first to explicitly state that she intends to stand. She resigned from the cabinet in protest over May's Brexit deal AFP/Getty Who could succeed Theresa May as Conservative leader? Sajid Javid Home secretary Sajid Javid is said to have a plan in place for a leadership race. He made headlines over Christmas when he declared that people smuggling over the English channel was a "major incident" and more recently when he revoked the citizenship of ISIS bride Shamima Begum. Son of a bus driver, he wants the Conservatives to be seen as the party of social mobility PA Who could succeed Theresa May as Conservative leader? Jeremy Hunt Foreign secretary Jeremy Hunt was recently thought to be the favourite in the event of a leadership race as he could sell himself as the man to unite the party. Critics worry that his long stint as health secretary could return to haunt him at a general election. He has reportedly been holding meetings with Tory MPs over breakfast to promote his leadership PA Who could succeed Theresa May as Conservative leader? Andrea Leadsom Following the Prime Minister's second defeat over her Brexit deal, Leader of the house Andrea Leadsom hosted a dinner party at which "leadership was the only topic of conversation", The Times heard. Leadsom ran against Theresa May in the 2016 leadership election before dropping out, allowing May to become Prime Minister AFP/Getty Who could succeed Theresa May as Conservative leader? Priti Patel Former international development secretary Priti Patel is thought to be positioning herself as a contender. One MP told The Independent "she knows she's from the right of the party, the part which is going to choose the next leader, so she's reminding everyone she's there." Patel left the government late in 2017 after it emerged that she had held undisclosed meetings with Israeli officials PA

But Phillip Lee, who quit Ms May’s government last year over her EU withdrawal policy, told The Independent that Mr Johnson would risk the Conservatives’ hold on power and the future of the party if he took it down a no-deal route.

It would be a “sweet irony” if the Brexit figurehead found himself forced to deliver a Final Say referendum because there was no majority in parliament for no deal, he said.

“I don’t think it is necessarily a personality who splits the party, I think the policy might,” said Mr Lee, who now chairs the Right to Vote group of Tory advocates of a referendum. “I don’t think a no-deal Brexit leaves the party intact. If that becomes the policy of the winning candidate, expect interesting times to follow.”

Mr Lee – who has faced threats of deselection in his Bracknell seat – said he had not personally decided whether he could remain in a party led by Mr Johnson, but he said it was an active topic of conversation among concerned MPs.

“Don’t think that these conversations are not taking place,” he said. “They have been taking place for at least a year.

“Anybody who thinks that people don’t feel passionately about stopping no-deal Brexit in the Conservative ranks and aren’t prepared to throw themselves under a bus doesn’t understand the situation.”

Mr Lee predicted “confidently” that at least 20 Tory MPs would opt for a second referendum if the only alternative was no deal; perhaps enough to secure a fresh public vote.

Phillip Lee MP predicts at least 20 Tory MPs would back a second referendum if no deal was the only alternative (Parliament Live)

When MPs voted on a referendum proposal in March it was defeated by 268 to 295. But Mr Lee said the vote was taken when Ms May’s “soft Brexit” alternative was still seen as a viable option.

The dramatic emergence of the Brexit Party under Nigel Farage had emboldened Tory Remainers by clarifying the options, he said. But he added: “Until soft Brexit 100 per cent disappears as an option, it remains a complicating factor.”

Labour MP Lloyd Russell-Moyle said he has had similar conversations: “There are many Conservatives who tell me that, when the crunch comes and if there is a more Brexity version put on the table by a new Brexity prime minister, they will come over to us on the confirmatory vote.

“We only need about 45-30, maybe, Conservatives to come over to us and we can achieve a majority in parliament.”

Although a no-deal Brexit is hugely popular among rank-and-file Conservatives, some of the party’s MPs are wary of a policy that might win them enhanced majorities in core Leave-voting areas like Lincolnshire but cost them seats in London, the southwest and Scotland.

Despite Mr Johnson’s declaration on Thursday and the collapse of cross-party Brexit talks on Friday, the race for the succession remains a phoney war.

Friends of the Uxbridge MP said he had not intended to use his appearance at a conference in Manchester as a stage to announce his intentions, but had simply decided to answer the question straightforwardly if he was asked if he would stand.

Few in Westminster will have missed the signs of rival camps being established in anticipation of a battle for the crown following Ms May’s fall. But despite the conversations in tea rooms and the “beauty parade” speeches by would-be contenders, the race is yet to begin in earnest.

“With it being a cold war for so long, everyone is up and ready, but no one is going to precipitate anything,” said one source.

Next week, Ms May will convene a meeting of the cabinet to consult on changes to be made to the Brexit Withdrawal Agreement Bill, including issues discussed with Labour in the cross-party talks which dramatically collapsed on Friday.

The prime minister will then deliver a speech setting out the details of the discussions, including a timetable for the introduction of the WAB in the House of Commons.

"The government has been negotiating with Labour for an agreement to build the biggest level of support across parliament for the Withdrawal Agreement Bill," a government source said.