Ed Miliband must commit to offering voters a referendum on membership of the European Union, a leading Labour donor has said.
Businessman John Mills is spearheading a campaign within Labour calling for a public vote on links with Brussels to be included in the party's election manifesto.
Mr Mills, the boss of home shopping firm JML, said it would be "unwise" to rule out a referendum and added that a commitment to a public vote would be popular with the electorate.
The Labour for a Referendum campaign has the public support of 15 MPs, but Mr Mills claimed there was wider "tacit" backing within the party.
He told BBC Radio 4's Today programme: "I think the Labour Party would be unwise to rule out having a referendum before the run-up to the next general election. A lot can happen over the next two years."
He added that he wanted a vote "the sooner the better" because "uncertainty about whether Britain is going to stay in the EU or not is bad for business and bad generally".
Mr Mills said he would vote to leave the EU in the present circumstances but backed Prime Minister David Cameron's plan for a renegotiation of the UK's relationship with Brussels followed by a referendum before then end of 2017, saying it was "not an unreasonable stance at all".
Labour leader Mr Miliband has criticised the Prime Minister's approach, claiming it was wrong to commit to a referendum now.
But Mr Mills said up to a third of Labour MPs would support a commitment to a referendum appearing in the manifesto "not least because the polls very clearly show that people do want to have the opportunity to take the decision on whether we stay in the EU or not".
The Labour for a Referendum group has the support of MPs including former Europe minister Keith Vaz, and high-profile backing from One Foot in the Grave actor Richard Wilson.
Mr Mills also raised concerns about Labour's economic policies, claiming none of the three main parties offered a clear solution to the UK's current problems.
He said: "There is a general problem in this country, which is that none of the parties I believe - the Conservative Party or the Lib Dems or the Labour Party - have got a very clear idea about how to get the economy to grow more robustly or to get unemployment down.
"I think that in the speech that Ed Balls made yesterday a number of proposals which are positive were produced, but I think that there is still a danger that we are going to finish up, if we are not very careful, with 10 years or so or perhaps longer, of austerity and no growth and this seems to me to be a very daunting prospect."
But Mr Mills said he supported Mr Miliband "very strongly" and would continue to back Labour.
"I support the Labour Party very broadly on large proportion of the policies it wants to pursue. I do have some disagreements about putting a commitment to a referendum in the next general election (manifesto) and I would like to do all I can and work with other people, if that can be possible, to see whether we can get a more successful prospect for the economy into Labour's next manifesto."