Ian Parsley has left the Alliance Party
Ian Parsley has resigned from the Alliance Party and joined the Conservative Party.
Mr Parsley said he had already given the party notice of his decision to quit his job as policy officer.
"The best means of delivering a shared future and a genuinely new type of politics would be through David Cameron's Conservative Party," he said.
He said he planned to stay on as a councillor in North Down and would likely be changing his designation.
He said he was mindful he had been elected by Alliance voters and would work closely with the party.
The North Down representative is beginning a new job with a Conservative-funded think-tank.
Alliance Party leader David Ford said he was "very disappointed" at the news.
He said he felt Mr Parsley had "left a promising political career to stumble down a blind alley".
Until Friday, Mr Parsley worked as an official with Alliance, but is set to begin a new post with the Centre for Social Justice, a think-tank set up by former Tory leader Iain Duncan-Smith.
Earlier, Mr Parsley dismissed speculation that he was about to defect.
However, his hand appeared to have been forced after the BBC saw an email which suggested he had been formally assessed as as a potential candidate for the Ulster Unionist-Conservative pact.
BBC political reporter Stephen Walker said: "I have certainly seen an email that was addressed to him and talked in such terms as though he was a would-be candidate.
Mr Ford said he felt any move to the Ulster Unionist-Conservative grouping would pose problems for Mr Parsley in the future.
"There are huge contradictions between the delusions of some Conservatives that think they are anti-sectarian, and the 1950s-style bigotry still coming from Ulster Unionist elements," he said.
"I fear Ian will find this contradiction more and more difficult to live with as time goes on."
Conservative Shadow Secretary of State for Northern Ireland Owen Paterson said the mood for change in Northern Ireland was "very strong".
"I am delighted that Ian Parsley, who is a rising star of the new generation of Northern Ireland politicians, has decided to join David Cameron's Conservative Party in order to help bring Northern Ireland into mainstream UK politics," he said.
There is also speculation that the Ulster Unionists and Conservatives want Mr Parsley to stand against their current North Down MP Sylvia Hermon in the upcoming Westminster election.
She has refused to endorse the link-up between the two parties and it is thought unlikely she will stand under the joint banner.
Mr Parsley, who was a member of the Conservative Party as a student, was the Alliance Party's European Election candidate earlier this year. He was first elected to North Down Borough Council in May 2005.