Tories who are flirting with voting Ukip should "come home" and vote Conservative because the party will "do more" on immigration and the EU, David Cameron has said.
Nigel Farage immediately rejected the call, insisting his backers had found a "new, more authentic home" after deserting both the Conservatives and Labour.
Cameron made the plea after one of his parliamentary candidates, Mike Whitehead, defected to Ukip, saying the eurosceptic party "stands up for local issues and allow people to stand up for their beliefs and principles".
While Ukip has threatened both Labour and the Conservatives, it poised to be a bigger threat to the Tories at the election, having already convinced two of Cameron's MPs to defect.
Cameron said: "I would say to those people that I totally understand the frustration people have felt about issues like immigration where they want more done, and we will do more.
"And I understand the frustration about Europe - where the country deserves a referendum - and with me as Prime Minister they'll get that referendum.
"But this election is not a time to send a message or make a protest. This election is about choosing the government of our country for the next five years, and the choice could not be starker between a Conservative government led by me, continuing with a plan that's working, and putting it all at risk."
Cameron said he had heard protests "loud and clear" but insisted the key choice for voters had to be between him and Miliband.
He said: "On the key choice about who has got the right team to run the economy, to keep your taxes down, to cut taxes further, to keep generating the jobs and the growth our country needs, come with us, come back home to us, rather than risk all of this good work being undone by Labour.
"That will be the message. That argument will become stronger and stronger because in the end we are choosing a government, we are choosing a team, and I say choose the team with the track record, don't risk it."
He added: "If you vote Ukip, if you vote for another minor party, you end up with the risk of Ed Miliband, propped up by Nicola Sturgeon driving him into even more spending, welfare, debt, bloat, all the problems. We've seen that clearly in recent days and during the television debate. That would be the effect and that's what we've got to avoid."
Farage said: "Neither former Labour nor Conservative voters who have switched to Ukip are going back.
"They've found a new, more authentic home, one in which they don't get roundly abused by their hosts."
Shadow minister Jon Trickett said: "David Cameron telling Ukip supporters that the Tory Party is their "home" is the clearest sign yet that the parties are natural bedfellows.
"The latest defection is another humiliation for David Cameron who, having failed to shoot Ukip's fox, is now is relying on their support."