Basil McCrea has warned that a united unionist candidate in the upcoming Mid Ulster by-election will simply drive up Sinn Fein's vote and leave a bitter, sectarian legacy.
The recently-resigned UUP member also accused his former leader, Mike Nesbitt, of going against the wishes of the majority of people in Northern Ireland by refusing to rule out more united unionist election candidates in the future.
Lagan Valley MLA Mr McCrea and John McCallister – Assembly member for South Down – quit the UUP last week after it teamed up with the DUP to support a joint candidate, Nigel Lutton, in the by-election.
Mr Nesbitt on Sunday said his party would consider similar moves with the DUP after the March 7 by-election, if results proved the controversial tactic a success.
"Let's see how the next three weeks go," Mr Nesbitt told the BBC's Sunday Politics show. "Will it work for the benefit of unionism? And when we get the result from that and the answer from that, we can look at it.
"It would be foolish to rule anything out at this stage. We've just started a process which to some extent I certainly view as an experiment –let's see how well it works for the benefit of the unionist people."
But Mr McCrea on Sunday night said greater unity between the two main unionist parties flew in the face of what most people actually wanted.
"I don't think that's good for Northern Ireland or what the majority of people want," he said.
The problem with unionist unity is that it doesn't work.
"I predict the unionist vote in Mid Ulster will fail and we will be left with the legacy of a very bitter, sectarian and emotional battle.
"Unionist unity will drive up the vote of Sinn Fein." In the wake of their resignations, Mr McCrea and Mr McCallister said they were not ruling out establishing a new political party, along with another former UUP MLA, David McClarty. Asked on Sunday night about the possibility of another party being formed by the three men, Mr McCrea replied that "discussions were progressing quite nicely".
The by-election was called to find a replacement MP for Martin McGuinness, who remains an MLA and Deputy First Minister.
Nigel Lutton was backed as a united unionist candidate in the Mid Ulster by-election following UUP and DUP constituency meetings. His father Frederick was murdered by the IRA in 1979.
He will be contesting the seat against Sinn Fein's Francie Molloy, the SDLP's Patsy McGlone and Alliance's Eric Bullick.
In 2007, DUP MP David Simpson used Parliamentary privilege to allege that Mr Molloy was involved in the murder – a claim he strongly denies.