Andy Burnham, frontrunner to succeed Ed Miliband, will propose that Labour sets up a separate campaign to keep Britain in the European Union.
The shadow health secretary, who is currently favourite to take over as Labour leader, believes that one of the factors in Labour's crushing defeat in Scotland was their campaigning alongside the Tories in the Scottish independence debate.
He said Labour will "learn the lessons" of the Scottish independence referendum, saying he intends to have a "separate Labour yes campaign".
He fears a similar result if seen to be campaigning with the Tories on Europe, making it harder to win back the support of Ukip voters.
Burnham visited the European parliament today. He has promised to shape EU reform - with the enforcement of a national minimum wage and addressing the undercutting of high-skilled labour.
Even though Labour is in a leadership campaign, I am not going to let the EU debate be defined by David Cameron. I will discuss with Labour colleagues in the European parliament what a distinctive pro-European reform package will look like.
These are areas that David Cameron will not be focusing on and that is why we be raising them today to make the Labour case for Europe. Renegotiation cannot be a green light to turn the clock back and weaken employment rights.
Chuka Umunna, shadow business secretary and former favour to succeed Miliband before pulling out of the race, told Progress magazine:
I think it is really important that, when making the argument to stay in, the Labour party is at the forefront of a broad, grassroots campaign involving a range of actors and groups in civil society.
We cannot be seen, or allow ourselves to be seen, as making the argument as part of a cosy club of established political parties and big businesses … we cannot allow Ukip and the Eurosceptic-right of the Tory party to frame the debate as one solely about immigration.