Sinn Fein has announced it is mounting an imminent legal challenge to Mr David Trimble's ban on its Ministers attending North/South Ministerial Council meetings.
The Ulster Unionists accused the party of engaging in "a protracted stunt" which would not improve the political situation.
Mr Gerry Adams yesterday said he had already given the Taoiseach, Mr Blair, and Mr Trimble private notification of his intentions. He said he expected the legal challenge to begin within 24 hours. It is understood that it will be made in the Belfast High Court.
Mr Adams said the action could be against Mr Trimble or the Northern Secretary, Mr Peter Mandelson, or both. He condemned Mr Mandelson for failing to intervene to save the institutions set up under the Belfast Agreement.
The move was condemned by the SDLP, which called on Sinn Fein to find a political way out of the crisis. An SDLP Assembly member, Mr Alex Attwood, said: "What is required is for us all to transcend party advantage and deal with political problems in a political way."
He called on all sides to fulfil their commitments under the agreement. The paramilitaries had to re-engage with the international decommissioning body; the ban on Sinn Fein Ministers must be lifted; and the British government had to implement proper policing reform.
The UUP senior negotiator, Sir Reg Empey, criticised Sinn Fein's legal challenge. "This action is entirely a self-defeating exercise. It will be counterproductive and will not help resolve the situation," he said.
"Two things can now happen. Sinn Fein will lose. If so, they will have proved unionism has a veto through the Belfast Agreement to block their participation in North-South Council meetings.
"Alternatively, if they win they will simply force the UUP to take even more serious action.
"Neither outcome helps resolve the situation. It merely underlines the lack of commitment by Sinn Fein/IRA towards achieving lasting peace and stability in these islands . . .
"When are they going to re-engage with the International Decommissioning Commission and begin the process of putting their weapons completely and verifiably beyond use?"
Sir Reg said it was serious allegations about Provisional IRA involvement in killings, gun-running and punishment attacks which had led to the UUP sanction.
At a press conference in Belfast Mr Adams said he had held lengthy talks with Mr Blair and Mr Ahern over the past fortnight to break the deadlock.
"We have tried to resolve this matter politically. We will continue in contact with both governments but I have little confidence in the British government's management of this process.
"The reality is that David Trimble is in breach of his obligations under the Good Friday agreement and Mr Mandelson is refusing to use his authority to defend the agreement.
"Sinn Fein is not prepared to tolerate this emasculation of the agreement by the First Minister, nor to acquiesce in the progressive termination of the All-Ireland Ministerial Council."
Mr Adams said that either his party or its Ministers, Mr Martin McGuinness and Ms Bairbre de Brun, could instigate the action.
"Our decision to take legal action, despite the lack of confidence by nationalists and republicans in the legal and judicial system here, is an indication of our grave dissatisfaction at how this British government, which promised so much, has pandered to a unionist veto," he said.
Meanwhile, the UUP Arts Minister, Mr Michael McGimpsey, said the international decommissioning body should be wound up next June if the Provisional IRA had made no move by then to put its weapons beyond use.
However, the Northern Secretary, Mr Mandelson, said the issue should be left to the chairman of the body, Gen John de Chastelain.
Mr Mandelson said that everyone had the right to take a case to the courts, but the current crisis was political and should be solved politically.