Mr David Burnside has said his selection as UUP South Antrim candidate for the next British general election reflects a hardening of Ulster Unionist opinion against the Belfast Agreement.
He defeated the party leadership's candidate, Mr Jim Wilson MLA, in Templepatrick on Friday night by 96 votes to 50 after the second count.
Mr Burnside yesterday described his defeat in the September by-election by the Rev William McCrea of the DUP as a "protest vote" and he was confident of regaining the seat.
A change in party leader was not needed but a "bottom line" on decommissioning must be established, he said. "More concessions is the last thing that's needed. We took all the risks and got nothing in return - that's not to say bravery was not shown. But I think opinion has hardened a lot and Friday's vote was very representative of that."
He said he recognised a majority of people favoured the continuation of a Stormont administration. However, its workings should be reviewed to make Executive ministers more accountable to the Assembly for their decisions.
"The recent decision by Martin McGuinness to abolish performance leagues on schools is an example of this lack of accountability," added Mr Burnside, a London-based businessman. Mr Burnside's victory angered one local pro-agreement UUP Assembly member, Mr Duncan Shipley Dalton.
"I am deeply, deeply disappointed by the decision made tonight," he said. "I think it is the wrong decision. I think he is a man who cannot win the seat and I think we have resigned South Antrim to four more years of Willie McCrea." Mr Burnside topped the poll in the first count with 70 votes. Mr Wilson, who received a special dispensation from party rules to hold more than one office to run for selection, received 44 votes, local councillor Roy Thompson, 29 votes, and constituency chairperson Ms Avril Swann was eliminated with 21 votes.
Mr Wilson, the UUP chief whip at the Assembly, said he was disappointed to lose the contest. Commenting on the result, Mr Peter Robinson of the DUP, described Mr Burnside as a "discredited and failed candidate".