* UKIP leader puts chance of "Brexit" at 30-50 percent
* Farage says chance of exit rises if Labour backs referendum
* UKIP rules out any alliance with French or Dutch eurosceptics
BRUSSELS, Feb 12 (Reuters) - If Britain's opposition Labour party backs a referendum on membership of the European Union the chances of Britain leaving the EU will greatly increase, UK Independence Party leader Nigel Farage said on Wednesday.
Conservative Prime Minister David Cameron has promised voters an "in or out" referendum on EU membership by the end of 2017. Labour is considering whether to match that pledge, which polls suggest the electorate wants.
"The single most important determinant of whether Britain leaves the European Union is whether (Labour leader) Ed Miliband changes his stance," Farage, a 49-year-old former commodities trader, told Reuters as part of a euro zone summit.
"If we can get Miliband to commit to a referendum in Labour's next general election manifesto, then the likelihood of a referendum happening shoots up way above 50 percent from where we are now."
Asked what the likelihood of Britain leaving the EU within the next five years was, the leader of the anti-EU party responded: "Better than a third, but not as a high as a half", before adding: "And growing."
Farage's party, once dismissed as a bit player in British politics, has seen its influence climb since 2009, when it came second behind the Conservatives in elections to the European Parliament. Since then it has challenged both the Conservatives and Labour for vacated British parliamentary seats and polled 23 percent in local elections in 2012.
The next test is the European elections in May, when UKIP is hoping it will be able to top both the Conservatives and Labour, although polls currently put Labour ahead, with UKIP and the Conservatives neck-and-neck for second place.
In the past, UKIP has stolen votes from Conservative supporters. But Farage said the aim was now to peel away disaffected Labour voters, particularly in northern industrial cities.
"What we've achieved so far is to shift the centre of gravity in British politics in terms of the overall debate," he said. "We've pushed the Conservatives into changing their position and I now want us to push to change the Labour party's position too."
If UKIP managed to beat Labour in the May polls, the pressure on Miliband to match the Conservatives' offer and neutralise the European issue would grow, Farage predicted.
"IN OR OUT"
The ultimate goal for UKIP is to get Britain to leave the European Union after more than 40 years of membership.
That prospect once seemed unimaginable but thanks to growing euroscepticism among the broader British public and Cameron's promise of a referendum, the possibility is now real.
Cameron has first to secure election victory next year and has then pledged to renegotiate Britain's relationship with the EU prior to offering a vote. Farage expected Labour to be the largest party after the 2015 election.
Perhaps even greater than the re-election challenge will be convincing the rest of the EU to let Britain change the terms of its membership - there is little appetite among member states for a fundamental revision of the EU's treaty.
Asked what he thought of the Conservatives' strategy, Farage was dismissive.
"They haven't got one," he said. "Cameron is the sort of politician that thinks, make a speech and that's it, it's all done and dusted.
"I don't see that they can get a treaty renegotiation on anything substantial.
"If after the European elections he starts to state what his renegotiation aims are, they will just be shot down," by the rest of the EU.
A bad EU election defeat for the Conservatives would lead to "panic and deeper splits", Farage, who left the Conservative party in the 1990s, said.
With 100 days to go before the European elections, Farage is hoping UKIP can show a late surge, as in the 2009 poll, that might allow it to grab around 20 percent of the vote, putting it either first or second.
That would also make it one of Europe's most influential anti-EU parties, alongside Marine Le Pen's National Front in France and Geert Wilders's PVV in the Netherlands.
Le Pen and Wilders have announced an alliance, saying they will work together in the same group in the European Parliament. But Farage rules out any cooperation with the ex-Fascist French party or anti-Islam Dutch group, and emphasizes that the eurosceptic parties are far from being united.
"I've ruled out alliances. Sitting in the middle of the eurosceptics, that's where I want to be."
(Additional reporting by Richard Mably and Paul Taylor)