The GMB trade union is polling its members about Brexit, as Labour campaigners for a second referendum look to trade unions to exert pressure on the leadership in the run-up to next month’s party conference.

Brexit is likely to be a contentious issue when Labour members gather in Liverpool. Campaigners say dozens of local parties have passed motions urging the leadership to back a vote on the final deal.

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In an email sent to GMB members, the general secretary, Tim Roache, wrote: “GMB wants to hear from members about your views on Brexit, and whether you think there should be a public vote when we know what the deal looks like.”

Labour MPs keen for their party to endorse a second referendum hope union leaders, who will meet at the TUC’s annual congress in Manchester, will send a message to the party leadership calling for voters be given the chance to reverse Brexit.

Separately, the grassroots pro-Corbyn movement, Momentum, is expected to ballot its members on Brexit in the coming weeks.

Mike Buckley, the director of Labour for a People’s Vote, which is organising constituency parties to send motions to the Labour conference, said: “As we approach conference, there’s clearly increasing pressure from the left of the party for a shift in the leadership position.”

Buckley’s group is urging constituency Labour parties to pass a motion calling for an immediate general election if parliament rejects the deal, and for that election to be fought on a manifesto that promises a second referendum, with remaining in the EU as an option.

It adds that Labour policy should be: “If we cannot get an election, campaign for a public vote and call for an election following a government defeat in that vote.”

Unite, whose general secretary, Len McCluskey, is a strong backer of the leadership, passed a motion at its policy conference last month supporting the “possibility” of a “popular vote” on Brexit.

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Several of the smaller unions, including the Royal College of Nursing and the steelworkers’ union, Community, have come out in support of a fresh referendum.

As negotiations between the government and the EU27 on an exit deal enter their final phase, the shadow Brexit secretary, Keir Starmer, has insisted the option of a referendum must be kept on the table if the deal is rejected by MPs.

However, the shadow international trade secretary, Barry Gardiner, last week suggested a second referendum could spark social unrest, saying: “If people feel that the route to change is no longer a democratic route, then you look to social disruption, perhaps civil disobedience, in a different way.”

Which motions reach the floor of Labour conference is determined through an arcane process by the conference arrangements committee.