By Guy Faulconbridge and Andrew Osborn
LONDON (Reuters) - Britain's opposition Labour party on Monday seized on divisions in the ruling Conservative party to suggest that David Cameron could be ousted as prime minister by his own party before the next election.
Ed Balls, who could be finance minister if Labour wins the election, said the issue pulling the Conservatives apart - membership of the European Union - would barely figure in voters' concerns come 2015.
The Conservatives' acute discomfort at being outpolled by the right-wing, anti-EU UKIP party in local elections last month has suggested to many Conservatives that UKIP could rob them of power in 2015, and allowed rivals to portray them as a party in disarray.
But Balls was the first senior Labour figure to suggest Cameron himself might be toppled before 2015, less than a full term after leading his party back to power after 13 years in the wilderness, albeit in a coalition government.
"They are very ruthless in the Conservative party ... so you definitely wouldn't rule it out," Balls told Reuters in an interview.
"The Conservative Party looks at him and thinks: one, he didn't win (in May) and, secondly, he didn't think he could win - and three years on, he's in a less auspicious position than he was then."
Speaking after an address in which he tried to convince voters that Labour was fit to run the economy, Balls said his own party was not beset by divisions on "big tactical issues" like the Conservatives.
"Is it likely the issue of Europe will be the most (important) or even in the top five issues at the next election? It's very, very unlikely," he said.
Although a small group of Conservative rebels openly want to oust Cameron immediately, few political analysts believe he could be toppled before the election.
Nevertheless, the pressure is growing, fuelled in the last few days alone by media reports of parliamentarians offering to use their influence for personal gain - albeit from Labour as well as the Conservatives - and suggesting a damaging romantic affair close to the government.
London Mayor Boris Johnson has made no secret of his ambition to succeed Cameron if the position became vacant, while at the same time pledging loyalty to the prime minister and his policy of holding a vote on EU membership.
Cameron promised in January to try to renegotiate the terms of Britain's EU membership and to hold a referendum on whether Britain should stay in the EU or leave if he wins in 2015.
Opinion polls suggest that many Britons - as well as the predominantly eurosceptic domestic press - view EU influence as an overbearing intrusion, and back Cameron's position.
But his pledge did not forestall big election gains in May by the UK Independence Party, which wants to lead Britain out of the 27-member bloc, or prevent a large, high-profile protest vote in parliament by his own eurosceptic wing.
Divisions over Europe helped to bring down two of Cameron's predecessors, Margaret Thatcher and John Major, while the three eurosceptic Conservative leaders who followed Major all failed to get into power.
"UKIP is driving a major split within the Conservative Party which may prevent the Conservative party ever, or certainly at the next election, being able to be a governing party," said Balls, a 46-year-old economist educated at Oxford and Harvard.
Labour under Balls' boss, Ed Miliband, has so far hedged its bets on a referendum and avoided a major public split over Europe, though a new Labour campaign group calling for a commitment to an "in/out" vote will be launched this week.
Asked if he believed Labour should promise before the 2015 election to hold such a referendum, he said: "No. That is not our position at the moment ...
"The answer to your question is: We are not advocating a referendum now, or deciding on a referendum, now but we are not setting our face against it because I think that actually has a real risk of looking out of touch."
Balls, best known for steering the 1997-2007 Labour governments of Tony Blair away from joining the European single currency, said the next election would be more about living standards, growth and public services than about Europe.
"All the experience of the last 20 years is that general election campaigns don't fundamentally turn out to be campaigns about Europe," he said.
Balls said he thought the economic arguments in favor of continued EU membership were strong, but that UKIP - which says it is also poaching Labour voters - was benefiting from voter anger at the established parties and the perceived elitism of those who supported EU membership.
"It is comfortable for us to be a pro-European party going into the next election," Balls said.
(Editing by Kevin Liffey)