But Nigel Farage's party has seen a surge in support following his two recent TV head-to-heads with Nick Clegg, the Europhile Deputy Prime Minister, which most pundits felt Ukip had won.
An Ipsos MORI poll of 1000 adults yesterday put the anti-EU party on 15% — four points up on last month — equalling its previous highest rating in the survey. Labour was up two points on 37%, the Tories down one on 31% while the Lib Dems slumped four to just 9%.
"Looking at the other parties, there are effectively two extremist camps," the Prime Minister said in his launch speech in Manchester. "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 poll at the weekend showed the Conservatives were on course to come third in the May 22 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; the Tories have promised an in/out poll by 2017.
In a rare reference to Ukip Mr Cameron said the anti-EU party could not be trusted.