Mike Nesbitt is expected to join the race to become UUP leader.
The Strangford MLA will formally launch his bid at Stormont on Thursday, UTV understands.
John McCallister has already announced his intentions to run and will be backed by his party colleague, Basil McCrea.
It is also expected that Regional Development Minister Danny Kennedy will also enter the three-man race - however he has not yet officially come forward.
Outgoing leader Tom Elliott, who was elected in 2010, revealed last week that he would step down at the end of the month.
Candidates wishing to succeed him have until Friday to hand in their nomination papers, ahead of the vote at the Ulster Unionists' AGM on 31 March.
Danny Kennedy will launch his campaign on Friday. He now says he has support of seven MLAs including himself.— Ken Reid (@KenReid_utv) March 14, 2012
Danny Kennedy will launch his campaign on Friday. He now says he has support of seven MLAs including himself.
— Ken Reid (@KenReid_utv) March 14, 2012