Moderate Tory MPs will help bring down an 'extremist' PM pursuing no-deal Brexit, predicts John McDonnell

Shadow chancellor argues ‘there may well be a majority in the House of Commons’ to stand firm – ‘faced with that situation’

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.”

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.

If the new leader failed at the hurdle, and no alternative government is confirmed by the Commons within 14 days, there will be a general election.