A People’s Vote on Brexit is “inevitable” if Labour cannot secure a general election in the wake of MPs rejecting Theresa May’s withdrawal plan, John McDonnell has suggested in a significant shift in the Opposition’s official policy.

The Shadow Chancellor’s remarks could help push the centre of political gravity at Westminster - should, as seems likely on current parliamentary arithmetic, the Prime Minister’s controversial Brexit Plan be defeated on December 11 - towards holding a second referendum.

The Labour leadership has stressed a sequence of events: defeating Mrs May’s plan and seeking a general election. If an election was not possible, then a People’s Vote was “one of a number of options”.

However, Mr McDonnell’s remarks suggested a second referendum in those circumstances was now Labour’s preferred option. Senior party figures like Sir Keir Starmer and Tom Watson have previously been warming to the People’s Vote option.

Labour MP Owen Smith described the Shadow Chancellor’s words as a “hugely significant acknowledgement” that securing a general election under the Fixed Term Parliaments Act was extremely difficult; one can only be brought about if two-thirds of MPs vote for one or the Government loses a confidence vote.

A source for the People’s Vote campaign said Mr McDonnell's shift was a “very positive” development.

Labour’s Stephen Doughty on behalf of the pro-EU Best for Britain campaign, also welcomed his comments. “It is clear if we can’t get a General Election, it is the will of Labour members and Labour voters across the country to put this question back to the people with the option to Remain."

However, during her visit to Scotland, Mrs May accused Mr McDonnell of trying to “frustrate Brexit” with his comments on a People’s Vote.

She accused Labour of wanting to “overturn the will of the British people” and added: “Parliament overwhelmingly gave the British people the vote. They voted to Leave. It's a matter of trust in politicians that they actually deliver on Brexit for the British people.”

In a BBC interview, the Shadow Chancellor said: “We want a deal that will protect jobs and the economy. If we can't achieve that - the Government can't achieve that - we should have a general election but that's very difficult to do because of the nature of the legislation that David Cameron brought forward.

"If that's not possible, we'll be calling upon the Government then to join us in a public vote. It's difficult to judge each stage but that's the sequence that we'll inevitably go through over this period."

Asked to clarify that he was saying it would, in those circumstances, be “inevitable” a second vote would take place, the Shadow Chancellor replied: "That's right. Our policy is: if we can't get a general election, then the other option which we've kept on the table is a People's Vote."

Earlier on Wednesday, Jeremy Corbyn’s spokesman related the previous Labour line that if a general election could not be achieved following rejection of the PM’s Brexit Plan, then a People’s Vote was “one of the options”.

Asked if the party leadership was “warming more” to a People’s Vote, the spokesman said since the party conference it had been an option. “But as Jeremy said it’s something for further down the track.”

On Tuesday, Nicola Sturgeon talked up the so-called Norway-Plus option, which would involve not only signing up to the rules of the single market, including freedom of movement, but also having a customs arrangement to provide frictionless trade; Norway is not in the customs union.

The First Minister claimed that Norway-Plus, which is predominantly in line with SNP thinking, was “gathering support” at Westminster.

But Mr Corbyn’s spokesman said the Norway option did “not work for Britain” while on Norway-Plus he said: “Anything that replicates the existing rules and regulations with no say would not work for the country. There would be a lack of democratic authority.”