David Cameron has urged potential Ukip voters to "come home" to the Conservatives on May 7.
The Prime Minister said the General Election was "not a time to send a message or make a protest" but to stop Ed Miliband and Labour forming a government.
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."
Nigel Farage has said Ukip is "becoming the opposition party" in northern England following the defection of Conservative candidate Mike Whitehead earlier today.
The Ukip leader denied Whitehead had been sacked by the Conservatives before he moved parties.
"He'd been having an argument in East Yorkshire with the Conservative Party," Farage said.
"I think when you boil it down what it really comes down to is this - in the north of England the Conservative Party are beginning to disappear the way they have in Scotland, and that Ukip is now becoming the opposition party in the north of England."
David Davis has accused Nigel Farage of making a misleading statement over the 'defection' of Conservative Mike Whitehead to Ukip.
Former Conservative leadership contender Davis said: "Given that Nigel Farage claims to be more straightforward than most conventional politicians, his tweet that he had "Just spoken to Tory Parliamentary candidate for Hull West and Hessle who has now left the Conservatives and joined UKIP" is at best disingenuous, and at worst the sort of misleading statement that he is always accusing others of making.
Former Conservative parliamentary candidate Mike Whitehead has claimed he was fired in a "pre-emptive strike" as the Tories knew he was about to join Ukip.
Whitehead, who announced his defection to Nigel Farage's party today, admitted he had already been removed as candidate for Hull West and Hessle but questioned the Tories' reasons for doing so.
He claimed the party already knew he was outside the Conservative group on the local East Riding Council when he was selected as the parliamentary candidate.
He told BBC Radio Five Live: "It's a bit odd, but I believe the reason they did that is because they knew I was standing up against the Tory group that was off side with the Conservative Party itself."
Of the email notifying him of his sacking last week, he said: "I got an email on Wednesday evening to say that if I did not stand aside from my intentions to stand in the local elections, they would remove my candidacy in West Hull and Hessle."
Ukip "defector" Mike Whitehead was told last week that he would not be selected as a Conservative candidate at the General Election, according to correspondence seen by ITV News.
The Conservatives say the email extracts - which they say were sent on 1 April - counter the claim that Whitehead was due to stand for the party in Hull West and Hessle, a Labour-held seat in East Yorkshire.
According to the Conservatives, Whitehead was told he would not be selected after refusing to support a local Conservative council candidate.
The defection of a Conservative councillor to Ukip is a "huge blow for David Cameron's authority", Labour has said.
The party's shadow minister without portfolio, Jon Trickett, said the move showed even the Prime Minister's candidates "know he cannot win a majority".
"Ukip and the Tories increasingly share the same people as well as the same policies," he said.
"Both stand for increased health service privatisation, extreme spending plans which threaten the NHS and further tax breaks for those at the top."
“David Cameron will not rule out working with UKIP, but this is clearly what his party wants. David Cameron must now come clean and tell us what his plans are to do a deal with Nigel Farage and UKIP.”
The Conservatives say a parliamentary candidate who "defected" to Ukip had already been sacked.
A spokesman for the Tories dismissed Mike Whitehead's announcement as "cynical, misleading and utterly calculating to try and score political points".
"He refused to support the local Conservative council candidate and so we wrote to him last week to say that his position was untenable and he could not stand for us at the general election.
"We were already selecting a new candidate for this constituency."
The defection of a Conservative parliamentary candidate to Ukip has dealt "another hammer blow to Tory pretensions in the north of England", Nigel Farage has said.
The Ukip leader said he was "delighted" to welcome Mike Whitehead - candidate for Hull West and Hessle - to the party, and said the move was proof "that today the real party of opposition to Labour in the north is Ukip".
A Conservative parliamentary candidate has defected to Ukip, the party has announced.
Mike Whitehead, an East Riding councillor standing for selection as MP for Hull West and Hessle, said he was "disgusted" with the behaviour of the ruling Tory group in East Yorkshire, and the "wilful refusal" of the national Conservative Party to intervene.
He also insisted the "secretive" council needed to be "opened up" and made more transparent".
Whitehead said he had been concerned about the behaviour of the controlling group of Tories since 2011, when seven Conservative councillors from Haltemprice and Howden resigned, claiming bullying and intimidation.
His parliamentary constituency is currently deemed a safe Labour territory, with former home secretary Alan Johnson defending a 5,700 majority from 2010, when the Conservatives finished third.
A helicopter has crashed into the sea near Flamborough, East Yorkshire, Humberside Police said.