Mr David Trimble is going on the offensive against Mr Jeffrey Donaldson to try to safeguard his leadership of the Ulster Unionist Party and see the full implementation of the Belfast Agreement, writes Gerry Moriarty Northern Editor.
He has made it clear that he wants to force the three dissident MPs out of the party.
The Taoiseach, Mr Ahern, meanwhile hopes to press ahead with efforts to revive the Northern elections, despite increasingly bitter tensions among unionists.
Mr Trimble postponed yesterday's planned meeting in Dublin with Mr Ahern as clear battle lines were drawn between the Donaldson and Trimble wings of the UUP after the Lagan Valley MP and two fellow MPs, Mr David Burnside and the Rev Martin Smyth, resigned the party whip at Westminster but said they would remain in the party.
The initiative has put great pressure on Mr Trimble's leadership and caused shock and despair among former Ulster Unionist Assembly members, who form the bedrock of Mr Trimble's support. One senior MLA warned that the UUP could be destroyed without a clear and speedy resolution of this battle between the pro- and anti-agreement wings of Ulster Unionism.
Mr Trimble, conscious of the threat to his leadership, has called a meeting of the UUP's party officers for Thursday or Friday to initiate disciplinary proceedings against the three MPs. The UUP leader made it clear that he wants to force them out of the party. The dissidents have equally indicated they are prepared for this showdown.
In relation to the MPs Mr Trimble spoke of "tidying up the situation". It appears that Mr Smyth as party president and Mr Donaldson as a party vice-president will be entitled to attend this officers' meeting.
Mr Trimble said the position of the president and vice-president was "wholly untenable". The three MPs could not remain "half in" the Ulster Unionist Party. "Resignation would clearly be the principled course of action. In any party elsewhere in the United Kingdom their action would be regarded as resignation from the party," he added.
Mr Trimble accused the MPs of repudiating the decision of last Monday's Ulster Unionist Council to support the UUP leader in his confrontation with Mr Donaldson. He said the MPs had made it clear that they would not "accept party democracy".
He also rebuked the MPs for proposing the creation of a loose coalition of anti-agreement Ulster Unionists, the DUP and other anti-agreement unionists.
"Those elements [the DUP and other No unionists] have amply proved their inability to offer constructive leadership to the unionist electorate and have as their primary objective the destruction of the Ulster Unionist Party, rather than the Union and the overall well-being of Northern Ireland," he said.
Mr Trimble, indicating that he wanted to see the agreement fully implemented, said yesterday was a "defining moment" for unionism as a whole and for Northern Ireland as well.
Mr Donaldson, Mr Burnside and Mr Smyth were equally damning in their criticism of Mr Trimble, suggesting that this could be the final battle between the utterly divided strands of the party.
Mr Burnside, however, was the only one of the three to clearly state that Mr Trimble should resign if he did not change party policy to theirs.
The DUP leader, the Rev Ian Paisley, described the MPs' resignation of the Westminster whip as "a gesture in the right direction" but added that many unionists would be disappointed that their action had not been more radical. He urged them to resign and join the DUP.