Ulster Unionist leadership contender Tom Elliott has said he does not believe there will be a single unionist party in his lifetime.
Mr Elliott was speaking at the launch of his leadership campaign.
He also said the formation of a single party "will only create polarisation" and a reduction in unionist votes.
Sir Reg Empey announced he would be standing down following a disappointing election during which the party failed to win a single seat.
On Wednesday, Mr Elliott also said he would not continue the electoral link with the Conservatives, arguing that the Ulster Unionist Party must not enter an election flying any colours other than its own.
He added:"When I hear calls from the DUP for unionist unity, and if they mean a single party, my answer is clear and simple - you will not see it under my leadership and I cannot see it in my own lifetime.
"That would only create polarisation and a further reduction in unionist votes.
"But if you mean unionist co-operation there may be opportunities to co-operate on some issues."
The Fermanagh and South Tyrone MLA's supporters included the MEP Jim Nicholson, health minister Michael McGimpsey and the party's deputy leader Danny Kennedy.
On Monday, the Lagan Valley MLA, Basil McCrea, confirmed he intended to enter the race to succeed Sir Reg as Ulster Unionist Party leader.
Mr McCrea said he could turn around the party's fortunes by promoting a strong united message and with changes to its assembly team.
Party members will meet on 22 September to select their new leader