A concerted attempt by Labour MPs and MEPs to engineer that their party would campaign unambiguously for a “confirmatory” Brexit referendum in the EU elections looks set to flop.

Instead Jeremy Corbyn’s preferred position of characterising a new public vote only as an option is likely to prevail, because he seems to have retained the backing of most of the leaders of the big trade unions.

The decision on how strongly to push for a referendum, and how Labour’s position on it should be worded in its manifesto, will be taken at a crunch emergency meeting of the party’s ruling NEC on Tuesday.

I am told by senior party sources that in talks last Tuesday with the leaders of the so-called five big trade unions - Unison, Unite, the GMB, Usdaw and the CWU - only the GMB signalled a strong preference for a confirmatory referendum to be upgraded from an option to a clear policy preference.

Unison and Usdaw are in theory aligned with the GMB on this, but sources close to Corbyn do not believe they will vote against the Labour leader’s preferred and more ambiguous referendum formulation in a couple of days.

As evidence, one source told me that Dave Prentis, General Secretary of the largest union, Unison, told Labour’s leader: “Jeremy, if you can get a deal with the Government then take it."

Unison leader Dave Prentis supports Corbyn's equivocal position on a public vote. Credit: PA

Although no one close to Corbyn thinks there is the remotest chance of negotiations between Labour and the Government on a Brexit pact reaching a successful conclusion, Prentis’s comments were interpreted as showing support for the Labour leadership’s equivocal position on a public vote.

“We think we have three-quarters of the votes on the NEC,” said an official close to the Labour leader’s office.

If this turns out to be so, it would be a blow to a majority of Labour members and probably a majority of its MPs too - who want a confirmatory referendum and who also believe that the party would win the largest share of votes in the EU elections if it in effect became the referendum party.

There will be a big final push by the party’s deputy leader Tom Watson and its Brexit spokesperson Keir Starmer (who is not on the NEC) to change Corbyn’s preferred manifesto wording so that it would simply call for a confirmatory referendum on any Brexit deal approved by Parliament.

Keir Starmer and Tom Watson want Labour's manifesto to call for a public vote. Credit: PA

Whether they can win the argument at the NEC will hinge to a large extent on how nine representatives of constituency Labour parties - essentially the representatives of members - choose to vote.

They know most Labour members want a referendum, including most members of its influential Momentum movement.

But some are reluctant to be disloyal to Corbyn by challenging his softer referendum position, and others are hard-left critics of the EU.

Possibly the most important protagonist at the meeting will be Momentum’s founder Jon Lansman, who is widely believed to be moving closer to Starmer’s and Watson’s position because he is aware that is what Momentum members prefer.

But a source close to Corbyn thinks Lansman does not see Corbyn’s referendum wording as “far” from a formulation he could support.

UPDATE

So I now hear Dave Prentis and the giant Unison union are in fact closer to Tom Watson than Corbyn on how Labour’s approach to a referendum should be characterised in its EU election manifesto.

In other words, at the emergency NEC meeting on Tuesday, Unison will back a form of words that makes it clear that a confirmatory referendum would be necessary in almost any practical circumstances of a Brexit deal being approved by MPs.

In fact, Prentis fears Corbyn misunderstood his position when he expressed it at that meeting with him last Tuesday. So just maybe Corbyn has not yet stitched up a victory for the more equivocal position on a referendum he prefers.