Labour cannot achieve its aims outside the European Union, Ed Miliband will say, as he warns the party must not "sit out" the referendum debate.
In what is being billed as his first major intervention since leading the party to defeat at the general election, he will appeal to the nine million voters who did back him in 2015 to vote "remain" on June 23.
Jeremy Corbyn, who has in the past expressed doubts about the values of EU membership, has been accused by critics of playing too low key a role in the remain campaign.
But Mr Miliband says his successor will enter the fray with a speech after the Easter break, with fellow ex-leader Gordon Brown also being lined up to play a central part.
Speaking at a Labour In for Britain event, Mr Miliband will say: "I want to send a very clear message to the nine million people who voted Labour at the last election: I believe the change you voted for, and still want to see in Britain, can only be achieved by us remaining in the European Union.
"And I believe that leaving would irreparably set back the cause of Labour politics.
"At heart our principle as a party is one of collectivism: the idea that we achieve more together than we can alone. It says it on our party card.
"So my argument for Europe is an argument rooted deep in Labour values of solidarity and co-operation."
He will say the debate must not be obscured by Conservative infighting on the issue.
"I want to say to all members of our party that we cannot sit this one out. In fact, quite the opposite.
"The last few days have shown the Conservative Party is divided, disunited and at each other's throats. But that makes it all the more important that we set out our case on Europe.
"The civil war in the Conservative Party cannot and must not obscure the central question in this referendum: are we more likely to secure social justice and progressive change inside the EU or outside.
"We cannot sit it out when this choice is so fundamental to helping build the kind of country we want. And we cannot sit it out when the decision of Labour voters will be so crucial to the outcome of this referendum.
"My speech today will be followed by one from our leader, Jeremy Corbyn, on Europe after Easter.
"We are united, we can speak with one voice and we will."
Leaving the EU would serve only those politicians with a "reactionary, pessimistic agenda", he will say.
And he will add: "Even if we leave aside the motivations of those who want us to leave, think of the implications of who it would help.
"Tax avoiders want to divide country from country to drive down tax rates. Polluters want to turn country against country in a race to the bottom on standards. Russia and those who disagree with us want to divide Europe.
"Outside the EU, that is what we would be exposed to."
It came as a new study found that leaving the EU would stunt the British economy even under the most "benign" post-Brexit deal, a think tank said.
Analysis of nine different outcomes by Oxford Economics found that "in any plausible scenario the UK economy is likely to be smaller by 2030".
It said the impact "could be limited" but only if there was little change to the status quo - such as if immigration was allowed to continue at a high rate and the UK still made contributions to the EU budget or remained part of the customs union.
Such policies, however, appeared "unlikely to be politically viable after a vote to leave", it said, calculating that the economy could be between 1.8% and 3.9% smaller under more realistic scenarios by 2030 than if the UK voted "remain" on June 23.
Pro-EU campaigners said the report exposed the "devastating" consequences of a No vote, but those pressing for a divorce from Brussels said it was based on "flawed" assumptions.
The report concluded: "In any scenario involving a significant clampdown on immigration, the overall fiscal position deteriorates markedly by 2030."
"This would require further annual tax rises or spending cuts equivalent to between £22 billion and £31 billion today, a fiscal hole that could be closed by raising VAT by between 4% to 6%."
Associate director Henry Worthington, who led the analysis, said: "The long-term impact of Brexit on the UK need not be severe.
"But benign scenarios involve retaining some of the least popular aspects of EU membership: continued high levels of immigration, restrictions on our ability to make trade deals with non-EU countries, and continuing to pay money to Brussels.
"Despite the short-term benefit to the UK budget of no longer contributing to Brussels, populist policy choices would damage tax revenues by much more. The result would be even more austerity."
The impact of Brexit on the rest of Europe "will be negligible in any scenario", the study found.