There were extraordinary scenes in central London today, as Douglas Carswell defected from the Conservative party and joined Ukip.
The prominent Conservative backbencher will trigger a by-election in his constituency of Clacton, where he has a majority of 12,068.
David Cameron branded the move "deeply regrettable" and "self-defeating".
Appearing at a joint press conference next to Nigel Farage, Carswell said: "I am standing here precisely because the Conservative leadership is not serious about change."
Carswell said he had written to his local association about his decision.
"I think many of them will come with me", he said.
Asked about how hard it was to come to the decision, he replied: "I've had more sleepless nights than you can imagine."
Farage said he had held defection talks with several Conservatives.
"It’s only a matter of time before one of them had the courage to come across," he said.
"There are a number of other MPs who hold views like Douglas. Let's see."
The Ukip leader said the move was the "bravest, noblest, most honourable thing I've seen in British politics in my life".
The defection was so secret not even Ukip press officers were not told of it until the shock announcement at a press conference this morning.
The by-election will cost the Tories about £100,000 to fight, but the damage Carswell has done to the party goes well above the financial.
As a popular backbencher who is well known for his politeness, popularity and independence, his move could tempt other Tory MPs to follow his lead.
Fellow eurosceptic Tory Norman Tebbit said Carswell was a "respected, upright, steadfast and brave politician who has always put his constituents and his country before himself".
But the fear that other Tory MPs may follow Carswell's lead settled down later when Liam Fox – seen by many as a figure-head for right wing Tories on the backbenches – went on TV to criticise his decision.
Nevertheless, any attacks made against Carswell by Tory HQ will be seen as an assault on one of the grassroot's favourite political figures, who commands more respect than most members of the party's high command.
Carswell went out of his way to criticise the way the Tory party is organised in his speech, saying a small clique of people were controlling the organisation from the top.
He focused on Sarah Wollaston, the Tory MP selected in an open primary, who was not elevated to Cabinet at the last reshuffle. It is widely believed she was overlooked because of her independent views.
Carswell's defection will also provide Cameron with a preview of what he might face if he goes into an in/out EU referendum campaigning for the UK to stay in, as his party faces being torn apart by ongoing fractures over Europe.
While Carswell was on the right of the Tory party and is critical of Europe and immigration, he was not considered one of the hard-right 'headbangers' who could be easily briefed against by Tory HQ.
"Ukip is not a backlash against the modern world," he said at the press conference.
"I'm an optimist. Britain is a much better place than it was when I grew up, a more tolerant place.
"The internet is making everyone a participant. As a father of a young daughter I've come to appreciate everything feminism has achieved. There's been a revolution in attitudes towards disabled people. What was once 'political correctness gone mad' we've now realised is just good manners. So much of Britain is so much better except for how we do politics."
There was uncertainty about whether Carswell really could be the Ukip candidate after it emerged that the party already had someone prepared to stand for the party.
Roger Lord, Ukip's existing candidate for the seat, told Buzzfeed: "As far as I'm concerned I'm carrying on.
"If Mr Carswell wants to join us then he can get in the queue and hand out leaflets with the rest of us."
But it later emerged that the party's rules allow the selection of a parliamentary candidate to be decided by its national executive council in consultation with the constituency committee in the case of by-elections.
.@DouglasCarswell is one of Parliament's greatest champions for democratic reform. I hope he is an MP after 2015, for whichever Party.— Zac Goldsmith (@ZacGoldsmith) August 28, 2014
If Parliament had more MPs like @DouglasCarswell , our democracy would flourish.— Zac Goldsmith (@ZacGoldsmith) August 28, 2014
I hope my Party resists the tribal impulse. @DouglasCarswell is a champion, a conviction politician, a true Parliamentarian.— Zac Goldsmith (@ZacGoldsmith) August 28, 2014
Douglas Carswell defection a big blow to Cameron. Highlights that distance between UKIP and Tory right is less than PM wants us to believe.— David Lammy (@DavidLammy) August 28, 2014
If he wins the by-election, Carswell would not actually be Ukip's first MP. That role went to Bob Spink, who defected in March 2008.
He is the fourth MP to trigger a by-election on switching party since 1832. The last was Bruce Douglas-Mann in 1982.
A Tory spokesperson called the decision "regrettable and frankly counterproductive".
"The Conservative party will contest the forthcoming by-election vigorously," they added.