Labour backbencher Frank Field is expected to warn that the party risks undermining the traditional Labour vote and losing voters to Ukip in "swathes", according to reports.
The MP for Birkenhead will say in the speech that the referendum could become a "suicide note" if the party backs the remain campaign.
It comes after Jeremy Corbyn said he would continue to make the case for Britain's membership of the EU.
Mr field, a former welfare minister, is expected to criticise open-door immigration policy as being damaging to wages and public services, the BBC reports.
"The last thing Jeremy needs to do is to undermine further the traditional Labour vote, much of which wishes to leave the European Union.
"For the party leader more actively to campaign for the Remain campaign will push even more Labour voters into the arms of Ukip," he will say.
But former cabinet minister Alan Johnson, who chairs the Labour In For Britain campaign, will warn that pro-Brexit Tories hope to use freedom from Brussels rules to scale back workplace rights to almost nothing.
In a speech to the Usdaw union conference, Mr Johnson will point to the fact that unions representing nearly four million workers have already pledged their support for continued EU membership.
"The vote in the referendum on the EU on June 23 is every bit as important as that election in July 1945," he will say in a reference to the reforming post-war government of Clement Attlee, famous for creating the NHS and expanding welfare.
"Perhaps more so. It is a vote about whether we remain or leave the EU, and there will be immediate consequences to that decision for everyone here, and every family in the land.
"It is vital that our unions campaign for Britain to remain in Europe and campaign for a Europe that protects working people and keeps the swivel-eyed alliance of the right of the Tory party and Ukip off our rights at work."
Ten unions, with just short of four million members between them, have announced they will support the Remain cause.
"From nurses and builders to railway workers, steel workers, postal workers and shop workers, trade unions will be campaigning for a Britain that remains in Europe," he will say.
"The rights of working people are protected by our EU membership, and Labour and our union movement are united in campaigning for Britain to remain in Europe.
"To protect the jobs that depend on our EU membership and the protections at work guaranteed through our EU membership, it is vital that our unions campaign for a Britain to remain in Europe.
"What about Michael Gove and Boris Johnson? Does anyone really believe they want to leave the EU because it will help working families?
"No, their vision is a small state with few, if any, workplace rights, and the Thatcherite "supply side" economy that Nigel Lawson was eulogising the other day.
"They know the EU protects workers' interests, and it's one of the principal reasons why they want to leave the EU.
"According to Michael Gove, we should emulate Albania, Boris Johnson says we should ignore the president of the United States because of his Kenyan ancestry, and Chris Grayling - a man who lights up a room when he leaves it - says that our country is never on the winning side whenever there's a vote at the Council of Ministers, when the facts show that we get our way on the vast majority of occasions."
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