The Queen should ask Jeremy Corbyn to form a government if MPs vote down Theresa May’s Brexit deal, John McDonnell has said.

The shadow chancellor claimed that, according to the UK constitution, Labour must be offered the chance to govern if Ms May is no longer able to command a majority in the Commons.

Until now, Labour’s policy has been that a general election should be triggered if the government’s proposed withdrawal agreement is rejected by parliament, but Mr McDonnell admitted this would be “difficult” to bring about.

Instead, he said, Mr Corbyn should be given the chance to put Labour’s plans before MPs, adding that he believed they would be backed by the Commons.

Ms May has faced questions over the status of her governing majority, with the Democratic Unionist Party, whose votes she relies on, refusing to support the prime minister unless she changes her Brexit plan.

Speaking to City leaders at an event organised by Reuters, Mr McDonnell said: “We haven’t sufficiently explored the concept of a minority government in our constitution, our custom and practice.

“If [the government] can’t command a majority, usually it is then the duty of the monarch to offer to the opposition the chance to form a government – and that would be a minority government – to see whether they have a majority in parliament.

“I think we could secure a majority in parliament for some of the proposals that we’re putting forward.”

Mr McDonnell was speaking after the DUP temporarily pulled its support for the government in protest at Ms May’s Brexit deal.

Cabinet ministers resign over Brexit deal Show all 13 1 /13 Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Esther McVey Britain's Work and Pensions Secretary Esther McVey arrives to attend the weekly meeting of the cabinet at 10 Downing Street in London. - Britain's Work and Pensions Secretary Esther McVey resigned from the cabinet over draft Brexit deal AFP/Getty Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @EstherMcVey1 of the resignation letter addressed to Prime Minister Theresa May from Works and Pensions Secretary Esther McVey. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Ms McVey has resigned, saying the Brexit deal âdoes not honour the result of the referendumâ. See PA story POLITICS Brexit. Photo credit should read: @DominicRaab/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Dominic Raab British Brexit Secretary Dominic Raab give a press conference at the end of the final round of talks in Brexit negotiations at the European Commission in Brussels, Belgium EPA Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @DominicRaab of the resignation letter addressed to Prime Minister Theresa May from Brexit Secretary Dominic Raab. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Mr Raab has resigned, saying he âcannot in good conscience support the terms proposed for our deal with the EUâ. See PA story POLITICS Brexit. Photo credit should read: @DominicRaab/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Rehman Chrishti Vice Chairman of the Conservative Party Rehman Chrishti tendered his resignation letter this afternoon PA Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Anne-Marie Trevelyan Parliamentary private secretary in the Department of Education Anne-Marie Trevelyan resigned stating that she cannot support the deal Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @annietrev of the resignation letter addressed to Prime Minister Theresa May from Anne-Marie Trevelyan, a parliamentary private secretary in the Department for Education. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Ms Trevelyan has resigned saying she cannot support the Brexit deal after negotiations âbuilt on the UK trying to appease the EUâ. See PA story POLITICS Brexit. Photo credit should read: @annietrev/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Sam Gyimah Universities minister Sam Gyimah resigned on November 30, claiming the government's decision to pull out of the EU's Galileo satellite navigation system as a deciding factor PA Cabinet ministers resign over Brexit deal Shailesh Vara Shailesh Vara who has quit as Minister of State for Northern Ireland, saying he cannot support Theresa May's Brexit agreement, which he said "leaves the UK in a halfway house with no time limit on when we will finally be a sovereign nation" PA Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Suella Braverman Brexit minister Suella Braverman has resigned, stating “It is not what the British people, or my constituents, voted for in 2016.” Cabinet ministers resign over Brexit deal Ranil Jayawardena Parliamentary private secretary to the ministry of justice Ranil Jayawardena resigned as he could not back the deal "in good conscience"

The Northern Irish party abstained on a number of key votes on the Finance Bill, throwing into doubt the future of its Commons pact with the Conservatives.

The prime minister will need the DUP’s votes if she is to have any chance of getting her proposed deal through parliament.

Mr McDonnell said the real test would come when MPs are given a “meaningful vote” on the Brexit deal next month.

He said: “The process around offering the opposition party [the chance to try to form a government] usually comes as a result of the minority government losing votes within the House.

“That’s already begun to happen on the Finance Bill, and the Finance Bill is usually the most significant piece of legislation in any year.

“However, I think the test is whether or not the government is losing consistent votes on the issue of the deal itself. If it is, then it’s demonstrating that as a minority government it hasn’t got the will of the House, the majority of the House, and therefore it’s a situation where it’s ungovernable, they’re not in government.

“Therefore the normal process is that the opposition party should be offered the opportunity to see if it can form a government that could secure a majority in the House for its proposals, and I think that’s the process we should go through”.

Turning to Brexit, the shadow chancellor said he was confident that parliament would block a no-deal outcome “at all costs”.

He said: “There is an overwhelming majority opposing anything that smacks of being no deal.

“I think that the concerns that people have of falling off a cliff edge – there is no majority for it. There will be a majority against it.”