More than 200 local branches of the Labour party are expected to renew pressure on Jeremy Corbyn to harden the party’s opposition to Brexit, debating a motion which activists hope will spread to more than half of all constituency parties.

Leftwing activists from the anti-Brexit campaign Another Europe is Possible, the group behind a similar push at Labour’s conference last year, have drafted a tough motion calling on Labour to support a fresh referendum.

The party’s policy cannot be changed again until its 2019 conference, but organisers hope that the sheer number of motions submitted before its member-led policy forum this Wednesday will have heavy symbolism in the crucial weeks ahead.

An unprecedented 119 constituency Labour parties (CLPs) submitted pro-referendum motions before Labour’s last conference in September. The campaign was key to setting the direction for Labour’s current policy – to push for an election but then leave all options on the table, including a referendum.

That number of anti-Brexit motions could almost double for Wednesday’s national policy forum; members from 201 CLPs have so far given commitments to submit pro-referendum motions for debate.

The forum will be attended by Labour frontbenchers including the shadow Brexit secretary, Keir Starmer, as well as party officials and union leaders.

Ana Oppenheim, an organiser for Another Europe is Possible, said: “The appetite at the grassroots to take a clear stand against Brexit is overwhelming. As the sequence of events unfolds in parliament, we need to look at the reality. Brexit is an attack on working class people, dripping with imperial nostalgia and migrant-bashing.

“If Labour fails to oppose it, we will lose millions of voters and, if it happens, any attempt to deliver a socialist programme will be hamstrung from day one. More and more party activists, especially on the left, are seeing this clearly. By the end of January, we reckon that a majority of CLPs could have debated a motion – and a big majority will pass it.”

Dozens of Labour members behind the drive are expected to offer stories of electoral difficulties that the party could face if it does not take a more explicitly anti-Brexit stance.

Sandy Paul, a Labour member in Poplar and Limehouse CLP, said: “Brexit is fundamentally the brainchild of the far right and disaster capitalists. For Labour to endorse any such project is to abandon its core socialist values of international solidarity, equality and justice.”

Another activist, Ruth Milsom from Sheffield Hallam CLP, said the party had to be “the voice of internationalism”, among a number who mentioned a desire for Labour to commit to protecting free movement. “We need to lead the fight against the Tory Brexit nightmare and not lag behind,” she said.

Steve Carver, a member in Bethnal Green and Bow CLP, said Brexit was not “an abstract political issue on which Labour can do any amount of tactical manoeuvring in the name of some bigger agenda”. Instead, he said, it was about “moral questions, issues of principle, and thousands of activists on the left are waking up to that.”

A study of Labour members found that 72% believed Corbyn should back a second referendum. The research, part of the Party Members Project led by Prof Tim Bale of Queen Mary University of London, found that while Labour members still strongly supported their leader overall, they appeared to be sceptical about his reasons for refusing to support a referendum.

However, the study also found a majority of Labour members said they supported the party’s current position, with 47% in favour versus 29% who opposed it.