Mr McDonnell’s remarks, made over two days at the Edinburgh Festival Fringe, have ignited a civil war within Labour ranks, and forced the party’s Scottish leader, Richard Leonard, to publicly reiterate his opposition to a second referendum.

Edinburgh South Labour MP Ian Murray called on Mr McDonnell to apologise to Mr Leonard, while a group of 13 prospective parliamentary candidates signed a public letter stating their hostility to another independence vote. Others supported Mr McDonnell, including MSP Neil Findlay, who said he was “entitled to his opinion” and “completely right”.

Yet despite being adamant that a Labour government would not refuse the granting a Section 30 order, and suggesting the current manifesto commitment against a second referendum could be scrapped, Mr McDonnell also said he would not rely on the support of the SNP to put Jeremy Corbyn into Downing Street, ruling out “any pacts or coalitions”.

Shadow chancellor John McDonnell during an interview with journalist Graham Spiers at the Stand's New Town Theatre for the Edinburgh Festival Fringe. Picture: Andrew Milligan/PA Wire

Speaking at an Edinburgh Fringe event, where he was interviewed by journalist Graham Spiers, Mr McDonnell said: “My view is we will not be blocking a proposal. The best way forward is to elect a Labour government, let us demonstrate what we can do, then I don’t think the Scottish people will be interested in another referendum.”

He said his position was backed by Mr Corbyn, and that there was now a “debate” around how the party should go forward in terms of dealing with SNP demands for a second referendum.

Current Labour policy is to oppose a second independence referendum, and Mr Leonard has previously said that a Labour government would not grant the Section 30 order needed for one to be held.

However, Mr McDonnell said such a position was being “set up” by First Minister Nicola Sturgeon, so she could argue the UK government was standing in the way of another referendum.

He said: “What I want is a Labour government, and let us demonstrate as a Labour government what we can do to transform people’s lives, and if, after a few years, people come back and say they want to test the water on an independence referendum then fair enough, that’s up to the Scottish people and the Scottish Parliament. I’m not here to block a democratic exercise by any means.

“For me, the priority is a Labour government. If the Scottish Parliament and the Scottish people determine that they want another referendum, I’m not being set up by Nicola Sturgeon because that’s what she’s trying to do. She’s trying to say it’s the big bad English yet again trying to prevent us holding a referendum.

“No we’re not. What we’re saying is it’s unnecessary. We will campaign against having a referendum, but we are not using parliamentary devices to block it — it’s as simple as that.”

Asked if he had changed his party policy “on the spot”, Mr McDonnell appeared to suggest the party was looking to change its current commitment to opposing a second referendum ahead of the next general election.

“In manifestos there’s always different views and we have a democratic process to decide it, and as we go forward in our discussions my view is we should argue against a referendum and concentrate on getting a Labour government elected, and we shouldn’t be set up by Nicola Sturgeon on this issue.

“That’s the debate we’re now having in how we go forward, and my view is that we shouldn’t be manipulated by Nicola Sturgeon in that referendum debate by trying to accuse a UK parliament of trying to block the will of the Scottish people – better for us to argue for the need for a Labour government and that a referendum doesn’t help the process.”

Mr McDonnell made the remarks despite a “conversation” with Mr Leonard who, he said, “understands where I’m coming from and is on the same page as me about achieving a Labour government.” But Mr Leonard said he had “put to him the very clear view that the people of Scotland do not want a second independence referendum and also to remind him that the last independence referendum was supposed to be once in a generation”.

He added: “I was elected 18 months ago directly by the members of the Scottish Labour Party, and one of the parts of the platform I stood on was clear opposition to a second independence referendum. So I think it’s pretty clear where I stand, it’s pretty clear where the membership of the Scottish Labour Party stand and that’s the view that we’ll be communicating.”

Labour’s Edinburgh South MP Ian Murray said Mr Leonard was right in his opposition to the granting of a referendum, “and deserves an apology from the shadow chancellor who contradicted him.

Mr Murray said: “John McDonnell should know better than to spout thoughtless, mealy mouthed ramblings about Scottish independence. Did he learn nothing from the 2014 campaign? Perhaps it’s because he was one of the few Labour MPs who didn’t come to help.”

Scottish Conservative leader Ruth Davidson said Mr McDonnell’s comments proved that Labour “would happily sell Scotland down the river if they thought it could give them a sniff of power.

“That is a rank betrayal of the two million Scots.”

Labour at war over split on Scottish independence vote