Corbyn warned his Brexit 'fudge' is causing Labour exodus as party prepares for key referendum talks

Jeremy Corbyn has been warned that his refusal to commit to a fresh Brexit referendum is causing Labour members to abandon to party in their droves, as its ruling body prepares to meet to decide the party's manifesto for next month's European Parliament elections.

A weekly meeting of Labour MPs was told that one constituency party had lost as many as 500 members over the "constructive ambiguity" in party's policy on a Final Say vote.

Mr Corbyn is facing mounting pressure to endorse a referendum on any Brexit deal passed by parliament ahead of a crunch meeting of Labour's National Executive Committee (NEC) on Tuesday. Members of the committee will attempt to thrash out the wording of Labour's policy on the matter, amid deep divisions between senior figures in the party.

Labour's deputy leader, Tom Watson, is leading calls for the party to endorse a public vote on any deal approved by MPs. That pits him against close allies of Mr Corbyn, who insist that it should only support a referendum on the government's Brexit deal or if one is needed to prevent a no-deal outcome.

As divisions on the executive committee spilled into public, Mr Watson urged Labour members to write to his fellow NEC members calling on them to support a Final Say vote.

He was backed by the leaders of four of Labour's main trade union affiliates: Unison, the GMB, the TSSA and Usdaw.

Labour MPs at the weekly meeting of the Parliamentary Labour Party (PLP) also said the NEC must clearly endorse a second referendum, while two spoke against. Bermondsey and Old Southwark MP Neil Coyle told the meeting his local party had lost hundreds of members over Mr Corbyn's refusal to support another poll in all circumstances.

Mr Coyle said: "I've lost 500 members since the peak at the end of 2016. That's not just because Brexit but the vast majority have left because of the European fudge. Members are saying they want us to oppose Brexit full stop, and they want a confirmatory vote as a minimum."

He said Labour's "constructive ambiguity" on Europe had led to the formation of The Independent Group and accused Mr Corbyn's team of failing to react to the creation of the new party.

The Independent is campaigning for a Final Say vote on any Brexit deal agreed by parliament.

118 Labour MPs and MEPs have now signed a letter urging the NEC to include in the manifesto a "clear commitment" to a public vote on any Brexit deal.

The letter says: "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."

In a separate letter, more than 30 Welsh MPs, assembly members, councillors and trade unionists urged the Welsh NEC representative to "unequivocally" back a manifesto "that includes a confirmatory vote on any Brexit deal agreed in this parliament, with an option to Remain."

More than 20 Labour European Parliament candidates also said they would campaign for another referendum regardless of the party's official policy.

As pressure mounted on the party leadership to fully endorse a Final Say vote, Mr Watson wrote on Twitter: "Labour member or supporter? Please let your reps on the NEC know if you want them to support a confirmatory ballot on a brexit deal in our Euro manifesto."

The comment prompted a backlash from some other NEC members supportive of Mr Corbyn. Claudia Webbe, who chairs the committee's disciplinary panel, told Mr Watson: “Our number one priority should be for a democratic socialist Labour government to deliver a sustained and irreversible shift in wealth and power in the UK.

“Supporting Jeremy Corbyn and the Labour Party to achieve this to kick out the Tories should be your focus.”

Another NEC member, Pete Willsman, mockingly compared Mr Watson to Ramsay McDonald, the first Labour prime minister, who became a figure of hate in the party after forming a government of national unity with the Conservatives.

Mr Willsman tweeted: "What is the difference between Ramsay MacDonald and Tommy Watson? Answer: the man from Lossiemouth had a firm grasp of loyalty."

But Mr Watson was backed by Momentum chairman Jon Lansman, who also sits on the NEC. He wrote on Twitter: "At conference we agreed: 'If the government is confident in negotiating a deal that working people, our economy and communities will benefit from they should not be afraid to put that deal to the public.’ So surely we too can all agree to a confirmatory vote on any government deal in our manifesto!”

Demands for the party to support a referendum on any Brexit deal were echoed by the general secretaries of four leading trade unions.

Unison's Dave Prentis said: “The prime minister’s deal and a no-deal Brexit would be terrible for the UK, public services and working people.

“A package that protects peace in Northern Ireland, keeps the UK in a customs union with a close relationship to the single market, and protects future employment rights isn’t on the table. That’s why any final proposal must be put back to the country for voters to decide.”

The GMB's Tim Roache told the Evening Standard: “It must be the public who have the final say on Brexit. A grubby political stitch-up to meet the needs of internal Tory party politics, rather than the best interests of our country, won’t put this question to bed for a minute, let alone a generation.”

TSSA general secretary Manuel Cortes said it was "time for our party to act on the overwhelming wishes of its members ", while Usdaw's Paddy Lillis said: “If our politicians are confident that their deal will benefit working people, our economy and communities across the country, then they should not be afraid to put that deal to the public.”