Defecting Conservative MP Douglas Carswell says he want to "shake up" the Westminster "clique" with his move to Ukip.
Carswell said the Conservatives' failure to take a stand against the European Union was at the heart of his decision.
ITV News Political Correspondent Libby Wiener reports:
Ed Miliband has tweeted that Douglas Carswell's defection show the Conservatives are "too divided to stand up for hard-working families".
Douglas Carswell’s defection is not just a blow to David Cameron. It shows the Tory Party too divided to stand up for hard-working families
A senior Tory in Douglas Carswell's Clacton constituency has said news of his defection to Ukip has come as a "bolt from the blue".
Mick Page, the leader of Tendring District Council, which covers Clacton said: "That's come as a shock - a bolt from the blue."
Speaking to the Press Association he added: "We always thought he was a Conservative very much against Europe but we thought he would work within the party to achieve his aims. As far as I'm aware the Conservative association had no warning of this."
A Conservative MP in Essex is calling for more clarity on Europe from the Tory leadership following the defection of Clacton MP Douglas Carswell to UKIP.
John Baron, the MP for Basildon and Billericay said: "I'm sorry that Douglas is leaving us. Every MP must make their own decisions about these matters but I do think it will yet again focus the Conservative Party and indeed the country on getting our relations right with the EU."
Mr Baron said he would stay in the Conservative Party but he is calling for more clarity on which powers the party would like to repatriate if they win the next election.
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 first Westminster MP.
he 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.
All this week we have been reporting on some of the key issues facing homeowners and would-be homeowners in this region in our 'Home Truths' series.
We put some of the issues to Housing Minister and Norfolk MP Brandon Lewis.
Click below to watch the full interview: