By Kylie MacLellan and William James
LONDON (Reuters) - British Prime Minister David Cameron described rival parties' views on the European Union as "extremist" on Thursday as he launched his bid to ward off a threat from an anti-EU party ahead of European Parliament elections.
Polls show the UK Independence Party (UKIP), which campaigns for tighter immigration rules and to leave the EU, could win the most votes in next month's European vote, pushing Cameron's Conservatives into third place behind the opposition Labour Party.
Under pressure from Eurosceptic lawmakers in his own party as well as a rise in support for UKIP, Cameron has promised to renegotiate Britain's ties with the EU and then hold an in/out referendum on the country's membership by the end of 2017 if his party wins a general election next year.
That puts him at odds with his pro-EU Deputy Prime Minister Nick Clegg, who has sought to position the issue as a two-way debate between his party, junior coalition partner the Liberal Democrats, and UKIP.
Clegg took on UKIP leader Nigel Farage in two recent TV debates on Europe, with polls from YouGov judging Farage the winner of both.
Cameron sought to place himself between the two.
"Looking at the other parties, there are effectively two extremist camps," Cameron said on Thursday in a speech to launch his party's European election campaign.
"One says 'We love the European Union'. The other says 'We hate the European Union'. One says 'We want things to stay the same'. The other says 'Let's just walk away from the EU'."
A YouGov opinion poll on Sunday found that the Conservatives were on course to come third in the European Parliament elections, behind UKIP and Labour which the survey suggested would battle it out for first place.
Labour has said it would only hold a referendum on Britain's membership of the EU if more powers were transferred to Brussels.
In a rare namecheck of UKIP, which has nine Members of the European Parliament (MEPs) but no elected seats in Britain's parliament, Cameron said the anti-EU party could not be trusted.
"When it comes to standing up for Britain in Europe, is there anyone you would trust less than a group of UKIP MEPs? They talk the talk in Britain - but as soon as they're on that plane to Brussels they change completely. When they do actually bother to vote, they don't stand up for Britain," he said.
"Labour and the Liberal Democrats won't give you a say on Europe. UKIP can't change a thing in Europe."
Cameron said only his party would pursue Britain's interests.
"You don't maximise those interests by saying everything's fine in Europe, you don't maximise those interests by walking away from Europe, you get the best for Britain by rolling up your sleeves and fighting for real change in Europe," he said.
UKIP, which styles itself as an anti-establishment party, hopes to benefit from an expenses scandal that forced culture minister Maria Miller to resign this week, saying Cameron's initial defence of the minister illustrated his remoteness from public opinion.
(Editing by Robin Pomeroy)