Mr David Trimble says he is confident his party's ruling body will approve the Stormont peace deal next weekend, despite strong opposition to it from several of its MPs.
The 800-strong Ulster Unionist Council will vote next Saturday on the agreement, following a recommendation from its executive to endorse it.
After a tense four-hour meeting at the weekend, the executive voted 55-23 in support of the deal, with 22 members absent.
Following the meeting, MPs Mr William Thompson and Mr William Ross both vowed to campaign against the agreement in advance of the referendum, which is due to be held on May 22nd.
Mr Thompson said a United Ulster Unionist Committee could be formed along with the Rev Ian Paisley, leader of the DUP, and Mr Robert McCartney, the UK Unionist Party leader, "to defeat this attack on our constitution".
He claimed that half the Ulster Unionist parliamentary party was opposed to the agreement. Mr Thompson, who represents West Tyrone, said he would fight the agreement from within the UUP.
"The UUP is a party of constituency associations which come under the Ulster Unionist Council. We will fight our battle through the association until we reject this document", he said.
Mr Ross, the East Derry MP, said he did not think the agreement was "sellable". He added: "I don't think that the unionist people will buy it whenever they understand what is actually in it, and in any case, when you look at the checks and balances that are in the internal structure within Northern Ireland, I think it's not workable anyway and will deadlock within a few days."
It is understood that the other dissenting MPs are Mr Roy Beggs, who represents Antrim East, Mr Clifford Forsythe, MP for Antrim South, and the Rev Martin Smyth, the Belfast South MP. Mr Forsythe and Mr Beggs could not be reached for comment yesterday and Mr Smyth is in Australia.
The Lagan Valley MP, Mr Jeffrey Donaldson, who is understood to disagree with parts of the agreement, declined to comment after Saturday's meeting.
Mr Trimble said he was encouraged by Mr Donaldson's "clear support" for the party's leadership and that he had undertaken not to engage in any attempt to divide the party.
The party's honorary secretary, Mr Reg Empey, accused opponents of the agreement inside and outside the party of being "hypocrites". He said: "Even some of those opposed to it in my own party and the DUP are all manoeuvring among their own colleagues for positions in the assembly, and this is the sort of hypocrisy that we see."
Speaking after the executive meeting, Mr Trimble said he shared many of his party members' reservations about the agreement and respected the views of those with "very deep objections" to it. "But", he added, "we have to take a judgment in the round and consider whether there is in this an opportunity for us to build for the future. We have taken that judgment in terms of the leadership of the party. That has been endorsed by the executive, and we go forward to the council meeting next Saturday with confidence."
Asked about the prospect of sitting in a devolved administration with Sinn Fein members, Mr Trimble said that this was an issue which caused "most concern to myself and others and will require effective action to be taken". He said he expected to see "significant developments" on the issue.
Mr Trimble emphasised that he had "no permanent or irreversible objection to Sinn Fein" as long as it renounced violence and accepted the democratic process. He said that talk of a split in his party was "unduly pessimistic".
Asked what his message to the people of the Republic was as they faced a referendum on Articles 2 and 3 of the Constitution, he said: "I'm sure many people in the Irish Republic have longed for the opportunity to get rid of a piece of old-fashioned furniture which reflected the thinking of the 1930s, if not earlier, and I would think that they would be glad to move into a better, more co-operative future which the British-Irish Council offers them in terms of developing a healthier relationship with all their neighbours in the United Kingdom."
