The Democratic Unionists will resign from the Stormont Executive, collapsing power-sharing in the North, unless the Assembly is suspended, First Minister and party leader Peter Robinson said.
The DUP ultimatum follows the arrest of three senior republicans over the murder of an IRA man in Belfast.
Mr Robinson said if other parties in the Assembly did not back the suspension move in a vote tomorrow, or if the British government did not act to suspend proceedings in the absence of that vote, then he and fellow ministers would quit.
“If that does not happen (the suspension vote), or as an alternative, the Secretary of State (Theresa Villiers) does not suspend the Assembly, then DUP ministerial resignations will follow immediately,” he said.
Peter Robinson announces if Stormont is not adjourned & or SoS doesn't suspend it then DUP ministers will resign pic.twitter.com/yGWosYPntQ— Tracey Magee (@Tracey_utv) September 9, 2015
The latest development in the political crisis sparked by the shooting of Kevin McGuigan came after Sinn Fein northern chairman Bobby Storey and two other senior republicans were arrested by detectives investigating the murder.
The police have said current members of the IRA were involved in the shooting - a revelation that has heaped pressure on Sinn Fein to explain why the supposedly defunct paramilitary organisation is still in existence.
The Ulster Unionists have already resigned from the Executive, claiming trust in Sinn Féin has been destroyed.
While the exit of one of the minor partners in the five-party coalition did not bring a collapse, if the DUP follows suit the institutions will fall.
The DUP wants the Assembly to be suspended until the McGuigan crisis is resolved.
The party will ask for a special meeting of the Assembly’s business committee to convene tomorrow to vote on suspension. With the SDLP and Sinn Féin unlikely to support such a move, the DUP would require the backing of the UUP and Alliance.
Mr Robinson added: “The DUP has made it clear it will not be involved in business as usual.
“Other parties must now step up to the mark and stop the Assembly from proceeding as if nothing has happened.
“We have attempted to create the space for these matters to be dealt with, but if others want the Assembly to function normally in spite of Sinn Féin’s position, we will have reached the point where, as a last resort, we will take this final step.”
If the committee does not vote for suspension, the focus will then shift to Ms Villiers, who could legislate to suspend the devolved administration.
Mr Robinson said Ms Villiers had not shown any inclination to take such action in her public comments to date.
He said if suspension does not occur he and his ministerial colleagues will walk away from the administration immediately.
Sinn Féin president Gerry Adams claimed competition for electoral support between the Ulster Unionists and Democratic Unionists was driving the crisis.
Mr Adams said: "Yesterday, I pointed out that the success of the current talks and the working of the institutions depended upon positive leadership.
"We are disappointed that that leadership is missing at this time.
"The suspension of the institutions will not serve the people who elected us or anyone else. All of this makes the talks process all the more challenging and urgent.
"What we need are working institutions, a real process of talks, and not a sham fight between unionists, if we are to resolve all of the outstanding issues and deliver for all in the community."