Trade union bosses have been warned there will be “hell to pay” from grassroots members if bloc votes force a “stitch-up” on Labour Brexit policy. 

Ahead of a major showdown at the party’s conference in Brighton, more than 600 trade unionists have signed a statement calling on their bosses to reject a Brexit compromise and back Remain.  

Jeremy Corbyn is on a collision course with the party’s pro-Remain rank and file and needs the support of union delegates to push through a key vote. 

The leader wants members to back a neutral position. He says that Labour would go into the next general election with a neutral stance pledging to negotiate a new Brexit deal with Brussels, which would be put to the public versus Remain in a new referendum. 

But his stance could be voted down if delegates from unions affiliated with the party refuse to back him and instead throw their weight behind a separate motion calling for a pro-Remain stance now. 

It represents a true crunch moment for the party leadership and follows a raft of senior figures, including John McDonnell, Tom Watson and Emily Thornberry, piling pressure on Corbyn to pick a side now. 

They fear Labour entering a snap general election campaign this autumn without a clear stance could see it steamrollered by the Lib Dems and the Brexit Party. 

But unions, such as Len McCluskey’s Unite, have been appealing to members to back Corbyn as they fear Leave constituencies could desert the party.

The trade unionists’ statement, organised by grassroots members just hours before the votes, underlines the anger they feel over the issue.  

Sue Rossiter, a GMB member and chair of Bethnal Green and Bow Constituency Labour Party, was among those who organised the statement, which tells union chiefs “members set policy” and “Brexit is about dividing working class people”. 

She said: “Union leaders need to understand that there will be hell to pay if they use their bloc votes in some kind of stitch-up against Labour members on Brexit. 

“Brexit is against our interests. It means lost jobs. We elect our leadership to represent us in the party, not sell us out in some sort of horse trade.” 

Mark Boothroyd, a nurse and Unite rep at St Thomas’ Hospital, who also signed the statement, issued a stark warning to union bosses.

She said: “Unions exist to fight for the rights of their members. 

“Brexit is a calculated move, cooked up by the Tory right, to undermine workers’ rights and attack migrants and free movement. 

“The whole trade union movement should be united in opposing it.” 

The Labour leadership could yet refuse to allow members to vote on the motions and instead decide the issue at a special conference on the party’s election manifesto. 

Michael Chessum, national organiser for the left-wing, pro-EU group Another Europe Is Possible, said Corbyn risked losing the vote on his Brexit compromise.

He said: “Ninety percent of motions to this conference are anti-Brexit, reflecting a membership which is overwhelmingly pro-Remain. 

“We are utterly determined to take a Remain position to the conference floor, where we are expecting a close vote. Using union bloc votes to defeat the overwhelming majority of members may well not work. 

“There will be an attempt to turn this into a loyalty test. But those proposing these motions are by and large people, like me, who have spent years fighting for the left inside Labour and backing Corbyn.” 

The motion put forward by Corbyn via the party’s ruling National Executive Committee is “logical”, McDonnell said on Monday. 

The shadow chancellor, who has said he will campaign for Remain in the promised referendum, insisted “there isn’t any war in the Labour Party” over the issue.

“What we’re saying is, when we know what the deal is, we’ll have a special conference and then determine our position,” he told BBC Radio 4’s Today.

“It’s very difficult for people to determine their position in advance of knowing the detail of that deal, but my view, actually I think, because I campaigned for Remain, I can’t see at the moment a better deal being achieved. And that’s my view.

“That’s why I’m saying I’m happy to go along with this logical sequence. And I’m happy for others to challenge me and say, ‘actually, no, this is a better deal’ – I’d like that debate. ”

The trade unionists’ statement reads: 

Downgrading migrants’ rights means making our members more precarious and exploitable, and less likely to organise. Brexit is about dividing working class people against each other.

The trade union movement and the Labour Party are nothing if they are not democratic. Members set policy – and Labour must back Remain.

We call on our unions, if they are affiliated to the Labour Party, to vote in favour of a Remain stance in any future public vote on Brexit.