Labour's backing for Britain's membership of the European risks driving millions of the party's "core voters" into the arms of Ukip, a senior backbencher has warned.
Frank Field, one of a handful of Labour MPs backing Brexit, said Jeremy Corbyn's decision to support Remain could be the "second longest suicide note" in the party's history after the infamous election manifesto of 1983.
"Since we had Tony Blair's mega election victory in 1997 we have lost 4.2 million voters. Many of those - maybe a million at the last election - went to Ukip," Mr Field told the BBC Radio 4 Today programme.
"They went because they thought we had a leadership which didn't represent their views about the nature of the country they wished to live in, the country they were born in. I think therefore the question about borders, and control over borders, is crucial to this referendum vote."
Mr Field said he believed Mr Corbyn - who had a long record of Euroscepticism - had only come out for Remain because he feared a challenge to his leadership if he declared for Leave.
"Every key vote that we've had on Europe since I've been in House of Commons and since Jeremy joined the House of Commons, we've actually been in the same lobby together, critical of Europe," he said.
"The truth is there is no threat to Jeremy's leadership because there is no alternative candidate. Therefore if it was done for that reason, it was done for very poor political reasons."
His comments came as former home secretary Alan Johnson, who is leading the Labour Remain campaign, prepared to issue a rallying call to unions to support EU membership as the best way of protecting workers' rights.
In a speech to the Usdaw union conference, Mr Johnson will compare the referendum vote to the 1945 general election that brought in Clement Attlee's post-war Labour government which created the NHS.
"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.
"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."
"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 Britain to remain in Europe."
He will accuse pro-Brexit Tories like Michael Gove and Boris Johnson of wanting to strip away workers' rights.
"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," he will say.
"They know the EU protects workers' interests, and it's one of the principal reasons why they want to leave the EU."