A definitive shift in Labour’s Brexit policy has been put on hold after the Unite union dug its heels in against moves to throw the party’s full weight behind a second referendum and a Remain vote.

Hopes that a change in position would be agreed on Tuesday at a crunch shadow cabinet meeting were dashed after the union’s intervention in talks with Jeremy Corbyn.

Senior shadow cabinet ministers including Tom Watson, the deputy leader and Sir Keir Starmer, the shadow Brexit secretary, will continue to press for full-throated support for a Final Say vote, but now believe they will have to wait longer for a breakthrough.

Labour supporters of a second referendum were encouraged by Mr Corbyn’s most forthright public support yet for a public vote on any Brexit deal.

Speaking in the House of Commons, the Labour leader said that Tory leadership contenders Boris Johnson and Jeremy Hunt have no “credible” plan for Brexit and “no mandate to force a disastrous hard-right Brexit on this country”.

He told MPs: “Whatever Brexit plan the new Tory leader comes up with, after three long years of failure, they should have the confidence to go back to the people and let them decide the future of this country.”

One shadow cabinet source noted that Mr Corbyn had brought up the referendum issue unprompted in the chamber, in response to a Theresa May statement on a different subject. “He didn’t have to say it,” said the source. “We are now firmly in the position of a referendum on any deal with Jeremy advocating it.”

Jeremy Corbyn (Reuters)

Mr Corbyn has been inching towards full support for a Final Say referendum since the disastrous European elections last month, in which Labour was beaten into third place behind the Brexit Party and Liberal Democrats on just 14 per cent.

He has told his MPs the party is “ready to support a public vote on any deal”, either in the form of a general election or referendum, and ordered a consultation on the policy. He last week indicated he is planning a speech on the issue, which People’s Vote supporters hope will confirm a move beyond the stance agreed at last year’s conference, which prioritises an election over a referendum.

It had been hoped that consensus on a clearer Brexit policy could be reached in talks with Labour’s union backers on Monday, for final approval at Tuesday’s shadow cabinet meeting.

But while unions including the GMB, Unison and Usdaw were in broad agreement on the need for Labour to be supportive of a referendum, Unite is understood to have set its face against a change in position.

Labour MPs said further delay in the party’s shift towards backing another referendum was an “abrogation of responsibility”.

Siobhain McDonagh, the MP for Mitcham and Morden, told The Independent: “We’re facing one of the biggest crises in our country’s history. We’re facing a no-deal Brexit that would lead not only to devastating consequences for our economy but also probably break-up of the United Kingdom, and we can’t make a decision.

“It’s been this tug of war that we’ve seen for the last six months. We get promised change and leadership but what we get is more delay and an abrogation of responsibility.”

Ilford North MP Wes Streeting said: “The arguments about our Brexit position are well-rehearsed and the position of the majority of Labour members, Labour voters and trade union members is clear: they voted Remain last time and want people to have the final say in a fresh referendum. We now need leadership. It’s the lack of clarity that is killing us.

John McDonnell (AFP)

Ahead of the union meeting, John McDonnell, the shadow chancellor, said he expected “white smoke” at Tuesday’s shadow cabinet meeting – in a reference to the Vatican signal that a new Pope has been chosen.

Speaking at an event in the City, Mr McDonnell repeated his view that it was now the “right time to go back to the people” – and he confirmed that he would campaign for Remain in another referendum.

And he said: ”Out of shadow cabinet tomorrow I’m hoping that a more definitive position will emerge.”