The Democratic Unionists will resign from the Stormont Executive, collapsing power-sharing in Northern Ireland, unless the Assembly is suspended, First Minister and party leader Peter Robinson said this afternoon.
The DUP ultimatum follows the arrest of three senior republicans over the murder of Kevin McGuigan in Belfast last month.
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.
Police have said current members of the Provisional IRA were involved in the McGuigan shooting - a revelation that has heaped pressure on Sinn Féin 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.
Tasoieach Enda Kenny is to meet with SDLP representatives at Leinster House tomorrow morning to discuss developments.
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.
McGuinness accuses unionists of leadership failure
Sinn Féin Deputy First Minister Martin McGuinness has accused the DUP and UUP of a failure of leadership. He said their actions were "foolhardy, foolish and totally devoid of the quality of leadership that is required".
"The demand, the ultimatum, that has been issued that the institutions be adjourned Sinn Fein is opposed to," he said.
"We are not going to jump to the tune of the inter-party rivalry that is being played out among both unionist parties at this time and I think it would be a grave mistake for the British government to suspend these institutions.
"I think it would send a very negative message and would be grist to the mill of those who in the past have tried to plunge us back to the past.
"I have spoken in recent days to the British government and I have spoken to the Taoiseach [Enda Kenny] this afternoon. David Cameron told me he would not suspend the institutions, the Taoiseach told me this afternoon he was totally opposed to the suspension of the institutions. So I think what is required over the course of the next number of hours and into tomorrow is a period of reflection for those who are involved in this inter-party rivalry."
UUP leader Mike Nesbitt said his party's position had been vindicated.
"There is one issue here - the fact the IRA still exists and Sinn Fein are in denial," he said.
"So there is only one fix and I cannot supply it, the DUP cannot supply it, for all their bluster. The only person who can supply the fix is Gerry Adams. Gerry Adams has to admit the IRA exists, with a structure. Without that, talks of adjournments and suspensions and talks - it's all bluster. It's down to Gerry Adams to tell the truth."
Cameron calls on politicians to work together
British Prime Minister David Cameron has appealed to politicians in Northern Ireland to think of the "nobler processes and the great noble principles that were put in place in the past" to help the region's crisis-hit political institutions.
He was responding to a question in parliament from DUP parliamentary group leader Nigel Dodds, who warned: "We have now reached the tipping point, indeed in my view we have gone beyond the tipping point."
Mr Dodds told MPs: "The Prime Minister will be aware that the situation in Northern Ireland - already grave following the IRA murder in August in Belfast - has escalated to new heights with the arrest today of the chairman of Sinn Féin in connection with that incident, and indeed other leading members of Sinn Féin. We warned about this earlier this week."
Mr Dodds warned of the "very grave state" of the devolution process unless action was taken.
He added: "We have now reached the tipping point, indeed in my view we have gone beyond the tipping point. The Prime Minister is aware the First Minister has met the Secretary of State this morning, he has put a proposal to her.
"Does he now accept that unless he and others take action that we are in a very grave state as far as devolution is concerned? We want to see government but only those committed to exclusively peaceful and democratic means can be in government.
"The people of Northern Ireland cannot be punished, it is Sinn Féin who should be dealt with. Does the Prime Minister agree?"
Responding, Mr Cameron acknowledged that "we are at a very difficult phase" of discussions in Northern Ireland.
He said there was no justification for paramilitary organisations and structures in Northern Ireland or anywhere else, adding they were a "blight on our society, they are not wanted, they should be disbanded on every occasion and on every side".
Mr Cameron said: "The only thing I would appeal to members in the DUP, the UUP, SDLP, the Sinn Féin members who don't take their seats in this House, as someone who sat on those benches and watched while the peace process was put together and the power-sharing arrangements were put in place, it was one of the most inspiring things that I've seen as a human being and a politician, to see politicians put aside their differences, put aside concerns about appalling things that have happened in the past and decide to work together.
"The appeal I would make to all of you is, please, have that spirit in mind, it was an amazing thing you all did in Northern Ireland when you formed that administration and that assembly.
"We'll do everything we can to help you but let us think of the nobler processes and the great noble principles that were put in place in the past, and let's do it again."
Minister for Foreign Affairs Charlie Flanagan this evening asked Northern Ireland's political leaders to pull back from the brink. He said: "I urge Northern Ireland's political leaders to pull back from the brink.
"A collapse of the power-sharing institutions will serve no useful purpose. The political talks now under way provide a critical opportunity to address genuine political concerns in a meaningful way and move forward positively in the best interests of the people of Northern Ireland."