Ulster Unionist MP Mr Jeffrey Donaldson announced today he was remaining in the party but resigning the whip in Westminister with immediate effect.
In a clear challenge to the leadership policies of Mr David Trimble, he was joined in resigning the whip by party president and South Belfast MP the Rev Martin Smyth and South Antrim MP David Burnside.
Mr Trimble has cancelled a meeting with the Taoiseach, Mr Ahern, because of the developing crisis within his party. He was due in Government Buildings in Dublin later today for talks with Mr Ahern on the peace process.
At a news conference in Belfast, Mr Donaldson and his two colleagues stood with former party leader Lord Molyneaux and said there had to be a realignment in unionism.
Mr Donaldson said: "This is not about civil war. This is about the principles our party was founded on".
Mr Burnside warned unless there was an about-face by the leadership of the Ulster Unionist Party, "the position of David Trimble is untenable".
The three men, who have long been opponents of the Belfast Agreement, are opposed to the joint declaration. They failed at a recent meeting of the Ulster Unionist Party's ruling council to get a majority to demand the party adopts an outright rejection of the declaration.
Since then, Mr Donaldson has been considering his position within the party, having strongly hinted that he may quit.
But today he made it clear he was staying and fighting Mr Trimble.
Mr Donaldson, Mr Smyth and Mr Burnside branded the joint declaration as being "packed with concessions to Sinn Féin's IRA" and offering little prospect ofdelivering on stable devolution or credible decommissioning.
They said it did not provide the best way forward for the greater number of people in Northern Ireland and was a product of "failed negotiation by those who have lost touch with their people".
They added: "It is clear that the leadership of our party does not represent, and cannot speak for, a growing majority of unionists and has comprehensivelyfailed to address their concern.
"These people need a voice and we will work with other unionists of a like mind in Parliament to ensure that their views are properly represented and theirconcerns adequately addressed," the trio said.The Ulster Unionist Assembly Party today expressed "deep regret" that its three Westminster colleagues found themselves unable to accept party policy agreed by a majority and the UUC meeting a week ago.
They accused them colleagues of showing "contempt for the UUC delegates".
In a statement issued after their regular weekly meeting, the assembly members called on all Ulster Unionists to recommit themselves to the task of bringing about and end to all paramilitarism and to "look to the wider interests of Northern Ireland as a whole as part of the United Kingdom".
Democratic Unionist Party leader the Rev Ian Paisley said the move by the three MPs was a "gesture in the right direction", but would not satisfy many unionists.
Many had been led to believe there would be a substantial and meaningful move and would be "very disappointed". He said in discussions with the MPs he would hope to convince them they would be better off joining his party.