A leading voice in the campaign to keep the UK in the European Union has admitted his opponents' supporters are more enthusiastic than those on his side of the argument.
Executive Director of The In Campaign Will Straw said that there was an "enthusiasm gap" in the EU debate at the moment.
And those committed to the UK leaving the EU were more likely to vote.
He was addressing a fringe event at the Labour Party Conference in Brighton.
Mr Straw, who stood and lost as a Labour candidate at the general election and is former Labour cabinet minister Jack Straw's son, said it would be a "huge challenge" to persuade the country to vote to stay in the EU, in the referendum David Cameron has promised before 2018.
'Horror film baddies'
He suggested that as things stood, roughly a third of the electorate were committed to the UK staying in the EU, a third wanted the UK to leave and a third were undecided.
Will Straw was one of several pro-EU speakers to voice his concern about his rivals articulating a more passionate message with a greater likelihood of resonating with voters.
Lady Royall, the former Labour leader in the House of Lords, said those making the case for the EU had to do so "as passionately and simply" as those advocating withdrawal.
Blair McDougall, the Labour activist who ran the No campaign in the Scottish independence referendum, argued that the very fact that an EU referendum was happening proved that on one level the pro-EU arguments that had been made up to now "hadn't worked".
He added that "populists and nationalists are very hard to beat. They are like baddies in a horror film. They keep coming back. For them it is about emotion and faith."
Labour MEP Jude Kirton Darling said "the minute we start talking about institutions we lose the argument," and reflected that politicians shouldn't be too prominent in the campaign.
"We're not trusted. Who would trust us?' she said.
Robert Oxley, the Campaign Director of Business for Britain, said those campaigning for the UK to stay in the European Union were "scared of talking about the EU. They are ashamed of the EU. They are trying to conflate Europe with the EU, and pretending their opponents want to raise up the drawbridge to the world."