Opponents of Mr David Trimble are mobilising against him and his stance of sharing power with Sinn Féin in the Stormont Executive.
Ms Arlene Foster, the party's honorary secretary, is to seek an Assembly nomination for next May's elections.
This follows a party decision to permit agreement sceptics, Mr Jeffrey Donaldson and Mr David Burnside, who are Mr Trimble's most outspoken party critics, to seek nominations also.
The two, who are both MPs, received permission at a meeting of party officials in Co Fermanagh to break with party policy and stand for a second elected assembly. Mr Trimble and Lord Kilclooney, the former Mr John Taylor, were granted similar permission before the 1998 election.
Mr Trimble's party critics, who have called a meeting of the ruling Ulster Unionist Council for September 21st, believe he has abandoned a party principle of sharing power with Sinn Féin only on condition of an end to republican violence. They feel no UUP Minister should sit at the Executive table until the IRA has been disbanded.
Mr Burnside said at the weekend that if selected he would stand on that principle, included in the party's manifesto commitments in the 1998 poll.
Ms Foster says the party needs to return to that election commitment: "I think that has been demonstrated given all that has happened over the past four years with IRA involvement in Colombia, in the Castlereagh break-in and in the sectarian rioting in Belfast."
Ms Foster narrowly lost out in the race to be UUP candidate in Fermanagh-South Tyrone at last year's Westminster election to the party chairman and Trimble loyalist, Mr James Cooper. He was defeated by Sinn Féin's Ms Michelle Gildernew by 53 votes.
Mr Trimble is in Johannesburg for the Earth summit. Yesterday, he met the Colombian vice-president, Mr Francisco Santos, to discuss the arrest of three Irishmen arrested on suspicion of links with FARC guerrillas in that country. The three are expected to go on trial next month.
The unionist leader said there was no doubt about links between the IRA and Colombian rebels. "That is something that the republican movement must address and only the republican movement can clear that up."
He told the BBC: "The longer they fail to tackle that issue head-on, the longer there will be difficulties in terms of public confidence in them." He was denounced for his remarks by Sinn Féin. Mr Gerry Kelly, the North Belfast Assembly member said the First Minister was "deliberately prejudicing the right to a fair trial of the three men".
Sinn Féin president Mr Gerry Adams said yesterday that UUP elements were working to create a crisis within the peace process to bring down the agreement. He said it was not a time for "pandering to the No men of unionism".
"The responsibility of all the pro-agreement parties, and this includes both governments, is to manage this process of change in a way which minimises the difficulties and avoids the potential for crisis."