Douglas Carswell has said he is leaving the Conservative Party because too much of the leadership "aren't serious about the change that Britain so desperately needs".
The Clacton MP says too many Conservatives "talk the talk" on key issues at elections but have not done enough to make change happen.
Tory MP Douglas Carswell revealed he had not told David Cameron he was defecting to Ukip.
Carswell "I've not yet spoken to the Prime Minister...if I'd phoned him up beforehand you'd have heard about it in another way"
The Prime Minister and Mr Carswell's relationship has not always been friendly, with Mr Cameron once telling the Clacton MP he needed to "get a sense of humour".
ITV News Political Correspondent Paul Brand is following updates from the special Ukip press conference at which Mr Carswell is speaking.
Carswell says he thinks members of his local party will come with him. But feels sad to be leaving them.
Clacton MP Douglas Carswell has said he will stand down as an MP and then stand for Ukip in the resulting by-election following his defection from the Tories.
He today announced that he would be leaving the Conservative Party, claiming the party's leadership are "not serious" about the change the UK needs.
If successful, Mr Carswell would become the eurosceptic party's only Westminster MP.
The Conservative MP for Clacton, Douglas Carswell, has used a press conference this morning to announce he is defecting to Ukip.
Mr Carswell, long a strong eurosceptic voice on the Tory back benches, made the announcement alongside Ukip leader Nigel Farage.
The Conservative MP for Clacton, Douglas Carswell, has defected to Ukip.
Nigel Farage told Good Morning Britain there is "not one ounce of complacency from me" after he was selected to stand in the South Thanet seat for Ukip at next year's general election.
"I'm not pretending for one moment that it's going to be easy, but Ukip is offering something different and distinctive," Farage said.
Asked if he would stand down as leader if the party fails to win a single seat at the election, Farage reiterated his comments from last night.
"If we'd failed in the European elections I would have stood down, if we fail next year the party will pick someone better than me, but do you know what? That ain't going to happen," he said.
UKIP leader Nigel Farage also moved a step closer to Parliament tonight, when he was formally chosen as the party's candidate for South Thanet at the general election.
Staking his position in the party on the line, he told supporters, UKIP would win seats next year or he will have failed as leader, and would quit.
ITV News Political Correspondent Carl Dinnen Reports:
The Labour Party has said that Ukip "doesn't share" Britain's values, as Nigel Farage has been announced as his party's choice to stand for Parliament in South Thanet, Kent.
Ukip don't share Britain's values. Whoever's name they put on the ballot paper, the truth about Nigel Farage and his party is that they would go even further than the Tories, with policies which would be bad for hardworking people. A vote for them is a vote for higher taxes for working families, huge giveaways for the rich, scrapping rights at work and charges to see your GP."
After accepting the Ukip local party officials nomination as candidate to stand in the South Thanet seat in next year's general election, Nigel Farage told the audience:
We can't for one moment afford to be complacent because we swept the board in 2013 and we swept the board here in 2014.
We did and that was magnificent. But don't think that the heavy artillery from the Conservative and Labour parties will not be fired on this constituency.
They will, they will, they will try very, very hard. They don't want Ukip breaking through into Parliament and they most certainly do not want me breaking through into Parliament.
Somebody said I'm David Cameron's worst nightmare. Well, that's not good enough. I want to be Ed Miliband's worst nightmare, too."
Following his speech, Mr Farage smiled broadly as he walked past branch members to applause and chants of "Nigel, Nigel, Nigel".
Ukip leader Nigel Farage has been selected by local party officials as their candidate to stand in the South Thanet seat in next year's general election