Ulster Unionist leader Mr David Trimble has piled fresh pressure on the British government to restore credibility to the talks process in Northern Ireland by acting against Sinn Féin over allegations of continuing IRA activity.
He has also called on the Democratic Unionist Party to take a similar "stand for peace and democracy".
Following talks with the Prime Minister, Mr Tony Blair, at 10 Downing Street yesterday, Mr Trimble accused London of "sending a desperate message to society . . . about people literally getting away with murder". He urged the DUP, Alliance Party and SDLP to "isolate" Sinn Féin.
Mr Trimble also spoke to the Taoiseach by phone yesterday.
Mr Ahern last night urged Mr Trimble to remain in the review process during the 15-minute phone conversation.
Mr Trimble told Mr Ahern of his reasons for withdrawing - his demand for sanctions against Sinn Féin over the alleged IRA attack on Mr Bobby Tohill - which the Taoiseach said he would consider.
The DUP yesterday ridiculed Mr Trimble's apparent withdrawal from the review of the Belfast Agreement.
The party's rebuff comes amid signs that the UUP leader's internal opponents are planning fresh moves to isolate Mr Trimble ahead of the annual meeting of the Ulster Unionist Council at the end of this month.
The Irish Times understands that those sponsoring a new "corporate" unionist leadership intend a fresh challenge to Mr Trimble's authority ahead of the party's a.g.m. in the hope of persuading former Stormont minister Sir Reg Empey to declare his willingness to seek the party leadership.
Sir Reg was alongside Mr Trimble at Downing Street yesterday, and is to travel with him to the US for the traditional St Patrick's Day festivities at the White House and in Washington.
However, while supporting Mr Trimble's assertion that the review process is "irrelevant" while the question of paramilitary activity is unresolved, Sir Reg refused to discuss speculation about the party leadership.
Mr Trimble said: "There is no point in continuing with the review when the underlying problem is not resolved, and that the republicans have not come to the point of a crucial decision to change towards peace and democracy.
"There is a duty on the Prime Minister to act - we have put that case to him. He is considering the matter. I hope he will act in a way that restores credibility to the process, but I cannot tell you at this stage whether he will."
There was the suggestion of a growing determination in Whitehall last night that paramilitarism has to be resolved, coupled with a resistance to "symbolic" or "knee-jerk" reactions.
One source said: "The question is how we sharpen the debate, but do so in a way which produces a result and not a pseudo-row."