Ed Miliband is resisting calls from the Unite union to change his stance on Europe, as he insists in a speech he believes Britain's focus "must not be on drifting toward exit".
The Labour leader has made clear he does not believe voters should be offered a chance to leave the European Union unless there are significant treaty changes.
His position has been challenged by Labour's main donor, the Unite union, which yesterday passed a motion at its annual conference calling on Miliband to shift his opinion.
"I have no truck with those who say we should cut ourselves off from the rest of the world," the Labour leader told an audience at the thinktank Policy Network's conference on Inclusive Prosperity this morning.
"Our focus must not be on drifting toward exit but on acting now to reform the European Union so it works for Britain.
"That means working with our allies to reform the things that aren’t right: the budget, rules on immigration and benefits, and giving more powers to national parliaments."
Unite general secretary Len McCluskey said yesterday the time had come for Miliband to reverse his position of "ducking this question" because doing so is a "vast hostage to fortune".
"I would not like to be Ed Miliband explaining why he is not joining other parties in offering the British people a vote on something that is clearly a growing source of public concern," he said.
"Without such a pledge our party will stand exposed. Ukip will be strengthened in some key constituencies. The Tories will hypocritically charge Labour with being anti-democratic. In a tight election this can make the difference."
Miliband will have relaxed after McCluskey confirmed Unite would provide significant funding to the Labour party to fight next year's general election.
But its dissatisfaction with the EU, which McCluskey claims has "danced to the bankers' tune", reflects little-discussed Labour faultlines on the issue.
The Labour Campaign for a Referendum group features MPs including Keith Vaz, Tom Watson, John Cryer, Frank Field and Graham Stringer.
Its chair, John Mills, said: "MPs, councillors, party activists and union members are all clear – Britain deserves a referendum on the EU question.
"The number of voices urging the Labour leadership to support an in/out referendum continues to grow as it is clear offering an in/out referendum pledge which will help secure a Labour government in 2015."
This week saw Conservative MP Bob Neill press ahead with his private member's bill seeking to guarantee a referendum in the next parliament.
Eurosceptic Tories will achieve their aim if the bill gets through the Commons unamended, which is expected to prove difficult to achieve.
But debates on the issue are expected to highlight Miliband's discomfort and are viewed on the government backbenches as a strong consolation prize for failure.
Miliband was left red-faced in the Commons yesterday after Tory MP Tony Baldry said the future Labour leader had distributed literature in his constituency which included calls for a referendum in 1983.
Today's speech is the first opportunity for Miliband to reply to that jibe. He added: "The government I want to lead will champion openness to the world because it is in our history and it is the way we have always succeeded.
"I will be a prime minister who seeks to build alliances for our vision of a reformed Europe."