Ian Parsley has resigned from the Conservative Party after describing the UCUNF project as "seriously flawed".
Mr Parsley left the Alliance Party to stand as a candidate in the general election for Ulster Conservatives and Unionists - New Force.
Mr Parsley said despite best intentions, the Conservatives lacked the political space and capacity to deliver effective politics in NI.
He also said he was "sorry" about the way he had left the Alliance Party.
"I regret not one second of my time spent in and with the Alliance Party, whereas I do regret wasting so much time and effort unproductively on the seriously flawed UCUNF project," he said.
"It is no doubt partly on the basis of how highly I spoke about the Alliance Party, even after I had left it, that my own parents chose to remain members, and that my partner has just become one."
Mr Parsley' s partner Paula Bradshaw quit the Ulster Unionists Party in early November after missing out on selection for next year's Assembly elections.
Ms Bradshaw stood under the joint Ulster Conservatives and Unionists' banner at the general election in May but has since joined the Alliance Party.