LONDON (Reuters) - British Labour leader Ed Miliband's chances of beating Prime Minister David Cameron at the next election in 2015 will be weakened unless he too promises a referendum on European Union membership, the party's biggest donor said.
Millionaire businessman John Mills, who made his fortune through a TV shopping company, is leading a campaign for the centre-left party to give voters their first say on whether Britain should stay in the 28-nation bloc since 1975.
Cameron, seeking to bridge deep rifts in his Conservative party and counter the rise of the anti-EU UK Independence Party, has pledged to negotiate to claw back more powers from Brussels, then hold a referendum on continued membership.
Miliband, who has a narrow lead over the Conservatives in the polls, has so far refused to match the pledge, saying it would cause years of uncertainty that would hurt an economy coming out of its worst crisis since the Great Depression.
While Labour is more pro-European and less divided than the Conservatives on the issue, it faces pressure from grassroots voters tempted by UKIP's Eurosceptical stance. Some Labour lawmakers say denying the British people a choice on the emotional issue could hurt the party.
Yet if Miliband were elected on a referendum pledge, he might find it harder than Cameron to win a plebiscite against likely Conservative as well as UKIP opposition, analysts say.
The Labour leader says he won't allow Britain to "sleepwalk towards the exit", a prospect that has alarmed the United States, Japan, European allies and many business leaders.
But Mills, a veteran campaigner on Europe who was the national agent of the "Out" side in the 1975 referendum, said ignoring voters' calls for a plebiscite would damage Labour's election hopes.
"Not having a commitment in the next Labour manifesto of some sort is going to prejudice the Labour chances," Mills told Reuters in a telephone interview on Monday.
"I can understand why the Labour leadership don't want to have a referendum, but I think they are going to have to weigh that up against the likelihood that a firm commitment not to have one would have a negative effect on the Labour vote."
An opera-lover who speaks French, German and Spanish and owns a house in the south of France, Mills, 75, says he is no "Little Englander". Instead, he says he is a Europhile who has long been frustrated by an EU he sees as an intrusive bureaucracy that stifles growth.
In 1975, Britain voted 2-1 to stay in the bloc which it had joined at the third attempt two years earlier. But that did not prevent decades of political squabbling over its ties with its biggest trading partner.
Since coming to power in a coalition in 2010, Cameron has faced repeated rebellions by Conservative Eurosceptics who want him to take a harder line on the EU or pull Britain out.
His promise in January to hold an in/out referendum by 2017 helped placate critics and avoid the rows over Europe that played a part in the downfall of Conservative predecessors John Major and Margaret Thatcher.
Mills, an Oxford graduate who has written several weighty economics books, cautioned against allowing Cameron to go into the election as the only major party leader offering an EU vote.
"Not everybody wants to come out, not everybody wants to stay in, but there is a very high feeling that some sort of democratic decision has to be taken," Mills said, without linking his financial support for Labour to the issue.
A YouGov poll for Channel 5 television in July found 61 percent of British voters supported a referendum. Asked how they would vote, 46 percent would choose to leave and 36 percent would opt to stay in.
On the face of it, Labour appears united on Europe. Only about 15 of Labour's 257 members in the 650-seat lower house of parliament support Mills' referendum campaign.
However, the party has had its share of trauma on the issue. Labour was split during the 1975 referendum, with senior Cabinet ministers allowed to join the "Out" campaign.
Disagreement over Europe was among the reasons that prompted a group of senior Labour figures to leave the party in 1981 to establish the centrist Social Democratic Party, forerunners of the pro-EU Liberal Democrats now in coalition with Cameron.
Labour campaigned to leave the bloc in the early 1980s, only to drift towards a pro-EU stance later. Three-time Labour Prime Minister Tony Blair was broadly pro-European, although he held back from joining the euro single currency.
Persistent chatter in Westminster suggests Labour's top team may be softening its opposition to a referendum.
Mills, who gave Labour shares worth 1.65 million pounds ($2.53 million) this year to become the party's biggest donor in the first quarter, said Miliband faces a tough choice and should not be rushed into matching Cameron on a referendum.
"He's in a difficult position," Mills said. "It makes sense to take a considered view rather than come to a snap judgement and then find you're in the wrong place."
(Editing by Guy Faulconbridge, Andrew Osborn and Paul Taylor)