Labour risks losing "a swathe" of voters to UKIP by campaigning to remain in the EU, one of its MPs has warned.
Frank Field said Labour's traditional voters had seen their wages, housing and services hit by "open-door" immigration from Eastern Europe.
He claimed 40% of them wanted to quit the EU - but their views were not being properly represented by the party.
Mr Field is among about a dozen - out of 229 - Labour MPs campaigning to leave the EU in 23 June's referendum.
The vast majority are part of Labour's remain campaign, headed by Alan Johnson, who on Tuesday claimed that a vote to stay in the EU would be as important as the election of Labour's reforming government in 1945, which led to the creation of the NHS and the welfare state.
"President Obama was right - being in the EU doesn't moderate British influence - it magnifies it," he said in a speech to shopworkers' union USDAW.
The former home secretary said the EU was not perfect but Labour had to fight for a Remain vote to keep "the swivel-eyed alliance of the right of the Tory Party, and UKIP, away from hard won workers' rights".
His message echoed that of Labour leader Jeremy Corbyn, who used his first major speech of the campaign to argue that leaving the EU would lead the Conservative government to launch a "bonfire" of workers rights.
Mr Corbyn, who has expressed Eurosceptic views in the past, insisted there was nothing "half-hearted" about his commitment to the Remain cause, amid criticism from Labour leave campaigners that he had only adopted the position to maintain party unity and did not really believe in it.
Mr Field said many of the workers' rights adopted by the EU had come from Britain, and he said the party's MPs were out of step with the opinions of many Labour voters.
"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."
The Birkenhead MP, who was welfare reform minister in Tony Blair's first government, suggested the EU referendum could turn out to be "the second-longest suicide note in Labour's history" - behind the manifesto that preceded its 1983 general election drubbing.
Mr Field has called in the past for Labour to replace its leader before the next general election but he told the BBC's Daily Politics he believed Mr Corbyn was now "secure" and talk of MPs overthrowing him was "absurd".
Elsewhere in the EU debate, the Commons Foreign Affairs Committee published what it said was a "reliable, unbiased analysis" of the issues involved.
The cross-party committee, which was divided on whether to back EU membership, said it had chosen not to endorse either side.
Instead it urged voters to consider the UK's trading relationship with the rest of the EU and the rest of the world, its "international representation and reputation" and how the EU and its policies might develop in the future.