Corbyn played down the prospect of reversing the UK’s departure from the EU and declined to confirm if he would back “remain” if there was a second referendum.

The comments in an interview yesterday were described as “unfathomable” by the SNP’s Westminster leader Ian Blackford after momentum appeared to have been building towards a second referendum in recent weeks.

“Jeremy Corbyn has finally come off the fence he’s been sat on for the past two years – but unfathomably he’s come down on the same side as Theresa May,” Blackford said.

SNP Westminster leader Ian Blackford. Picture: House of Commons/PA Wire

“The Labour Party is incapable of providing opposition to the worst UK Government that most people can remember.

“The SNP, meantime, will grasp any opportunity to see Scotland’s wishes respected. We are the party working with others, trying to find a way to remain in the EU through another referendum.

“Jeremy Corbyn seems incapable of leading a party worthy of opposition, never mind government.

“He is the midwife to the delivery of the Tories’ Brexit plans.

“The SNP will provide that much-needed strong voice, standing up for Scotland, while Labour are posted missing on the biggest political challenge facing our country.”

Labour is seeking an early general election if – as expected – the Commons rejects May’s Brexit deal in a vote in the new year. But the party has also left open the option of supporting a second referendum.

But Corbyn said if he becomes Prime Minister he would “go back and negotiate and see what the timetable would be” with Brussels.

Asked about Labour’s position if there was a fresh popular vote, he told the paper: “It would be a matter for the party to decide what the policy would be, but my proposal at this moment is that we go forward, trying to get a customs union with the EU, in which we would be able to be proper trading partners.”

Last week Work and Pensions Secretary Amber Rudd said that a second referendum could be the most plausible scenario if the Commons rejects May’s deal and cannot agree an alternative.

A cross-party coalition of opposition parties at Westminster, including the SNP, Liberal Democrats, Greens and Plaid Cymru, back a so-called People’s Vote.

Liberal Democrat leader Sir Vince Cable said Corbyn “refuses once again to take the blinkers off”.

He added: “He is ignoring the concerns of his own supporters and the economic damage experts warn Brexit will do to the UK economy.

“On Brexit, you simply cannot put a cigarette paper between Theresa May and Jeremy Corbyn.”

Jeremy Corbyn's pledge to go ahead with Brexit is '˜unfathomable', says SNP