The Ulster Unionist Party leader, Mr David Trimble, told reporters that his Assembly team would be boycotting the formal nomination of ministers to an executive minutes before the executive was to be formed.
He said he still wanted the Belfast Agreement to work rather than to "crash". The period of the summer between now and October when the review is due to begin should be used productively to allow progress to be made, he said.
Mr Trimble again called for a republican move on decommissioning. Despite the UUP refusal to nominate ministers he advised against a general feeling of hopelessness. "Things are still possible," he said.
The UUP leader, with his full Assembly complement, including
the anti-Belfast Agreement member Mr Peter Weir, beside him, made his announcement outside party headquarters in Glengall Street in central Belfast shortly before 10.30 am.
As six miles away at Stormont the other parties prepared to go through the academic process of dealing with ministerial nominations Mr Trimble was telling journalists that he would not be going to Stormont, and would not be nominating ministers.
"There will not be an inclusive organisation formed today, and Sinn Fein will not be going into the administration today. This renders the proceedings at Stormont unnecessary," said Mr Trimble.
Equally, he said, the initial DUP motion on yesterday's order of business seeking the exclusion of Sinn Fein from any executive would not be debated because the DUP did not have the 30 signatures necessary for such a motion.
The DUP with support from other anti-agreement unionists had 29 signatures up to yesterday, but it emerged later that Mr Weir, who had lost the party whip, had withdrawn his name.
Mr Trimble - indicating that Mr Weir was now back within the party fold even though Mr Weir stated yesterday he had not sought the return of the whip - stressed that all the UUP's 28 Assembly members were now acting in concert.
Mr Trimble said the expulsion motion would have been a charade. "The only thing such a debate would do would be in fact to widen and deepen division between the parties, that it would only sour the atmosphere. We think it would be wrong for the atmosphere to be soured in that way," he said.
"We considered it better that the Assembly proceedings be wound up as quickly and as quietly as possible and on as positive a basis as possible."
But the agreement should be suspended rather than collapsed, he said. "The process should not be crashed. But instead it should be parked. Those who arranged today's business were designing a crash, and that is not in the interest of the people of Northern Ireland.
"We do not want to leave a message of hopelessness. We want to leave a message that things are still possible, that it can still succeed, and even if we have to park for a while then things can still move forward."
In the subsequent review the key parties would be the UUP, the SDLP, the DUP and SF, he said.
Addressing Sinn Fein he said: "We know that you are a part of this process. We know that your involvement is necessary for a successful outcome. But you must know that it won't work unless you address the issue of peace and weapons.
"We know it won't work without you, you must know it won't work without your decommissioning. Many people within the republican movement now realise this, but I think time is necessary for more to realise this, and time is necessary for it to begin and for it to happen."
Later Mr Trimble, defended his decision to boycott the sitting of the Northern Ireland Assembly and his refusal to nominate ministers. A review was now the "obvious way forward", he said.
The request from Mr Seamus Mallon, for him to resign from his position as First Minister-designate, was "unfortunate", he said, but he gave no indication of intention to step down.
The resignation of Mr Mallon was "a loss for us all". Mr Trimble said he was seeking advice on his own position and highlighted the presence of a "caretaker" provision in the Belfast Agreement in the event of the resignation of either the first or deputy first minister.
"The question that I ask myself is not what is best for me but what is best for the process." Ulster Unionists wanted to make it work.
The aim was to enhance the credibility of the process, not to damage it. According to Mr Trimble the running of the d'Hondt mechanism for the allocation of ministries, in the absence of the UUP, had damaged the peace process.
"There are some people in government, and in some parties, particularly in Sinn Fein, that seem to think that the Ulster Unionist Party can be forced to act, and we decided to make it clear in the clearest possible terms - we will not be forced."
He put the blame for yesterday's "charade" in the chamber on those who had pushed for the premature formation of the executive and set a "premature" deadline.
"I think that those now who were responsible for pushing the issue of d'Hondt and for creating the farce realise that they have made a serious mistake. Wiser counsels knew it was going to end badly."
A review was the way forward, he said, and his party would enter it with a positive approach.
In boycotting the Assembly sitting the UUP had prevented a potentially "acrimonious" debate on the exclusion of Sinn Fein from government, Mr Trimble claimed.
"What we have got to do is to build this process in a way that will work, not crash the way it did today; not be destroyed, as it would have been had I acted otherwise."
When questioned if he intended to give back the Nobel Peace Prize, Mr Trimble said he would not return it because he had "earned it".