LABOUR’S ruling body is coming under intense pressure from MPs to end the so-called “constructive ambiguity” on backing a public vote on a Brexit deal and include a "clear commitment" to do so in its European elections manifesto.

The move comes as a row broke out after the party produced a draft campaign leaflet for the May 23 poll without any reference to a second EU referendum.

Last autumn at its party conference Labour agreed to a policy of forcing a general election if Westminster voted down the UK Government's deal or talks ended in a no-deal outcome.

However, if neither of these developments happen, then Labour says it "must support all options remaining on the table, including campaigning for a public vote".

Sir Keir Starmer, the Shadow Brexit Secretary, was said to have been “furious” when it emerged the draft campaign material for the Euro poll failed to mention a second referendum. It is thought the leaflet is now being hastily rewritten.

But on Tuesday, the NEC meets to draw up Labour’s Euro poll manifesto. Several MPs have signed a letter calling on the leadership to ensure "without caveats" the party’s campaign was "the only viable alternative to Nigel Farage's Brexit Party".

The letter says: "Our members need to feel supported on doorsteps by a clear manifesto that marks us out as the only viable alternative to Nigel Farage's Brexit Party.

"We need a message of hope and solidarity and we need to campaign for it without caveats. To motivate our supporters and to do the right thing by our members and our policy, a clear commitment to a confirmatory public vote on any Brexit deal must be part of our European election manifesto.

"We understand the many different pressures and views within our movement, but without this clear commitment, we fear that our electoral coalition could fall apart," it adds.

Pro-Remain Scottish Labour MP Ian Murray told The Herald: “Next week’s NEC should confirm the unanimously agreed Labour Party policy that any deal that Parliament agrees should go to a confirmatory vote.

“That’s the policy that’s been consistently empathised at the despatch box by both Jeremy Corbyn and Keir Starmer. Anything short of this would be a disloyal to the overwhelming majority of Scottish Labour members and supporters.

“The time for constructive ambiguity is over and the NEC must now deliver,” declared Mr Murray.

He also seized on a Survation poll, which suggested 67 per cent of Scots now wanted to stay in EU, up from 62 per cent in the 2016 referendum.

The Edinburgh South MP noted: “In next month’s European elections, it is vital Scottish Labour campaigns on an unequivocal pro-EU platform. That means we must show full-throated support for a People’s vote on whatever Brexit deal is finally agreed.

“Scottish Labour has an opportunity to lead from the front on this and represent the majority of Scots who want to remain in both the UK and the EU.”

Meanwhile, Alistair Campbell, Tony Blair’s former communications chief, warned that if there were no commitment to a public vote in the Labour manifesto, then, for the first time, he might not vote for the party.

Speaking on the BBC's Political Thinking with Nick Robinson podcast, the Scot said: "Whatever emerges from the process going on now is going to be so different from what was promised, it has to go back to the people for a confirmatory vote.

"Now I would like it if they [Labour] would then say, if that were to happen, the Labour Party would campaign vigorously to stay in the European Union. But the first bit's fine for me. If they don't do that first bit, then I'd find it very difficult to vote Labour and that would be the first time in my life."

In other developments:

*the Eurobarometer survey of Europe-wide public opinion found in the UK 45 per cent of people would vote to stay in the EU compared to 37 per cent, who would vote to leave; 18 per cent were undecided and

*Sir Vince Cable, speaking at the launch of the Liberal Democrats’ EU election candidates in east London, said he regretted failing to pull together a Remain coalition and form a “common platform” with other like-minded parties on Brexit.