Moderate Tory MPs will help Labour bring down an “extremist” new prime minister pursuing a no-deal Brexit, John McDonnell has predicted.

The shadow chancellor – as he announced Theresa May’s successor would face an immediate vote-of-no-confidence – said there could be a “majority” in the Commons for a general election, or a Final Say referendum, in those circumstances.

“We will be talking to the other political parties,” Mr McDonnell said. Asked if that included Conservative backbenchers, he replied: “Yes.”

“This isn’t a matter of asking people to be disloyal to their beliefs or their party,” he told the BBC.

“We’re now possibly faced with an extremist leader of the Conservative Party coming in, willing to take us over the edge of a no deal.”

Mr McDonnell said: “Faced with that situation, I think there may well be a majority in the House of Commons to bring about some form of public vote – and that could include a general election.”

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

The comments came after Rory Stewart became the first senior Tory to rule out serving under Boris Johnson, describing the no-deal Brexit he is willing to pursue as “damaging and dishonest”.

Within hours of the prime minister’s resignation, the leadership favourite ruled out a further Article 50 extension, telling an audience: “We will leave the EU on 31 October, deal or no deal.”

Leading pro-EU Tories, including Dominic Grieve, the former attorney general, have hinted at a willingness to use any means to prevent a crash-out Brexit.

In the interview, Mr McDonnell also said Labour should move quickly to agree a new Brexit policy, after widespread criticism that the party’s existing stance is muddled, saying: “The world has changed.”

On the no-deal threat, he added: “Most of the analysis means that will lose people their jobs, undermine their livelihoods.

“We can’t stand by and let an extremist Conservative leader take their country down with them.”

The timing of the Tory leadership election, to conclude only days before Westminster’s summer break in late July, risks a constitutional crisis, some experts believe.

The new prime minister – particularly one chosen on a no-deal ticket – is likely to be challenged to prove that they have a Commons majority within days of taking office.

As well as opposition from anti-no-deal Tories, he or she must also retain the support of the Democratic Unionist Party that has propped up the May administration.

Following the defection of three Tories to The Independent Group, now Change UK, the working majority of the Conservative-DUP alliance is just six – meaning only four Tories would need to switch sides for a no-confidence vote to succeed.