Tory MP Mark Reckless has said he is leaving his party to join UKIP, announcing his decision on the eve of the Conservatives' conference.
The Rochester and Strood MP told the UKIP conference he had resigned as an MP, triggering a by-election.
He said that, as a Tory, he could not keep a promise to "cut immigration while treating people fairly".
He is the second Tory MP to defect to UKIP after Clacton MP Douglas Carswell did the same in August.
Mr Reckless will seek re-election in his constituency for UKIP.
Mr Reckless won his seat as a Conservative in 2010 with a majority of 9,953 votes over Labour's Teresa Murray.
'Best and brightest'
Appearing on stage at the UKIP conference in Doncaster, Mr Reckless said: "Today I am leaving the Conservative party and joining UKIP."
He said he had not taken the decision lightly.
Mr Reckless said the Conservative leadership was "part of the problem that is holding our country back".
Criticising the three main parties, he said: "People feel ignored, taken for granted, over-taxed, over-regulated, ripped off and lied to."
On immigration, Mr Reckless said constituents needed to believe that the UK had control over who comes into the country, adding: "At the moment we do not have any sense of that."
He said: "Does anyone left or right genuinely support an immigration system where we turn away the best and brightest from our Commonwealth, people with links and family here in order to make room for unskilled immigration from southern and eastern Europe.
"I promise to cut immigration while treating people fairly and humanely.
"I cannot keep that promise as a Conservative. I can keep it as UKIP."
By Chris Mason, political correspondent
So lightning really has struck twice for the Conservatives.
Another month, another defection and one timed to inflict a morale-sapping blow on Tory activists gathering in Birmingham for their conference.
Keeping a secret at Westminster is no easy task and yet Nigel Farage has managed it twice - engineering defections without anyone finding out until the big moment of their announcement.
That skilled choreography will unnerve Conservatives further because it'll leave them asking one big question: Who's next?
UKIP's prospective parliamentary candidate for Rochester and Strood Mark Hanson, meanwhile, said he had been unaware of the plans to install Mr Reckless in the seat, adding he would now "stand aside".
UKIP leader Nigel Farage said his new party member had "shown huge courage", adding: "He has thrown his lot in with us and we will do everything we can to get him elected."
Asked about further possible defections, he said: "Of course there are Conservatives I am talking to but there are Labour people too.
"There are Labour people who are deeply frustrated with Ed Miliband's leadership."
BBC South East political editor Louise Stewart said Mr Reckless had told her recently he was unlikely to stand for UKIP - unless the party put up a candidate to stand against him.
Andrew Mackness, chairman of Rochester and Strood Conservatives, said he was "astonished and disgusted" by Mr Reckless's decision.
"He has misled the hardworking people of Rochester and Strood who voted for him," he said.
And Conservative Party chairman Grant Shapps said: "I think the surprising thing is Mark says he wants lower taxes, immigration control, he says he wants a referendum on in/out, on Europe.
"Well that's exactly what we are planning to deliver and so I think this decision of his is completely illogical.
"It makes it slightly harder to deliver that decision, rather than easier."
Labour's shadow minister for the cabinet office, Michael Dugher, meanwhile, said: "This is a hammer blow to David Cameron's already weakened authority.
"On the eve of his conference we again see that Conservatives' confidence in Cameron is plummeting. David Cameron has always pandered to his right, and even they are now deserting him."
Mr Carswell, who defected from the Tories to UKIP in August, said he was delighted Mr Reckless had joined UKIP, saying he had "done the honourable thing" in letting his electorate decide.
Mr Carswell will himself stand against seven other candidates for the Clacton seat in a by-election on 9 October.
BBC political correspondent Robin Brant said it was a "nightmare scenario for the Conservative party", a scene that the Conservative leadership "hoped they would never see".