David Cameron has warned voters that Nigel Farage and the UK Independence Party (Ukip) cannot be trusted in European elections next month.
The Prime Minister branded Ukip "extremists" and insisted they would not "stand up" for Britain's interests in Brussels.
But he also admitted to being worried that "apathy" could play a major part in the result.
The comments came as Mr Cameron launched the Tories' campaign for the local and Euro contests on May 22, and tried to draw a line under the damaging expenses row that claimed Maria Miller's scalp.
The Conservative leader argued that his was the only party with a "clear plan" - "r enegotiate, get the best deal for Britain, then put the decision to the British people".
"Looking at the other parties, there are effectively two extremist camps," he said.
"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'.
"But in all this there are three words they are missing, three words that sum up everything we are about in Europe.
"Britain's. National. Interest. British jobs. British opportunities. British livelihoods.
"And our argument is 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."
Turning his fire specifically on Mr Farage, who is aiming to drain votes from the Tories and top the poll, the premier went on: "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 indeed their own leader has said they 'cannot change a thing in Brussels'."
Interviewed earlier on BBC Radio Norfolk, Mr Cameron said "apathy" was his main fear in the Euro contest. He insisted he had already managed to bring in reforms in the EU, such as cutting the budget and vetoing a treaty that was not in Britain's interests.
"There is no opposition that worries me. What worries me is people being apathetic and thinking you can't change things in Europe," he said.
Mr Cameron was trying to bounce back after a torrid week that eventually saw him jettison his culture secretary under pressure from backbenchers.
In another setback, one of the party's vice-chairman, Michael Fabricant - who has previously mooted an electoral pact with Ukip - was sacked last night.
The Lichfield MP said he refused a request from Conservative chairman Grant Shapps to quit after his prominent condemnation of Mrs Miller and opposition to the HS2 rail project.
The Prime Minister was forced to admit in the Commons that he had underestimated the continuing depth of public anger about expenses, five years after the original scandal over abuses broke.
Mrs Miller's departure defused a potentially hostile meeting between Mr Cameron and Tory backbenchers yesterday evening.
A senior Tory source said the mood in the 1922 Committee was one of "relief". MPs had been up in arms about the leader's refusal to despatch Mrs Miller, complaining that they were being berated by voters when out campaigning.
Mr Cameron told the gathering the party now needed to concentrate on four big fights - the local and European elections, the Scottish referendum and the general election.
"The Budget shows when we talk about the economy we can do it," he said.
A ComRes poll for ITV News suggested that Mr Cameron's public standing may have been weakened by his handling of the affair.
Some 63% thought he had dealt with the problem badly, compared to just 8% who said he had done well.
The figures were little better among Tory voters, with 50% critical and 15% believing he had performed adequately.
Labour leader Ed Miliband accused the Prime Minister of a "terrible error of judgment" in failing to sack Mrs Miller immediately after the publication of last week's Commons Standards Committee report.
And Ukip leader Nigel Farage, who travelled to Mrs Miller's Basingstoke seat in a sign that his party will target a high-profile scalp in next year's election, said: "I think it reflects very badly on David Cameron that he has completely misjudged the public mood on this."