The Ulster Unionist Party is pulling out of the Stormont Executive to form an opposition - but insists it does not want to wreck devolution.
Leader Mike Nesbitt stressed the move was entirely related to the killing of Kevin McGuigan in which the PSNI has said Provisional IRA members were involved.
Flanked by his most senior party colleagues yesterday, the UUP chief denied political posturing to appear more hardline than the DUP, or that he was "dismantling" the Stormont administration.
However, it puts pressure on the DUP, which is meeting Secretary of State Theresa Villiers this afternoon, to follow suit, which would block the Executive from functioning and plunge the devolved Government into crisis.
But Mr Nesbitt argued Sinn Fein's denial over the alleged involvement of PIRA members in the killing had driven a hole through the Good Friday Agreement.
"Sinn Fein has no credibility, we have no trust, and without trust we have nothing.
"The IRA need to go away and stop terrorising their own communities," he said.
Seventeen years on from the Good Friday Agreement, Mr Nesbitt said police had confirmed the Provisionals still exist and have a command stucture and its members commit murder.
He added: "We are in a bad place but this can be fixed. We need some clarity about the IRA and its command structure."
He suggested it was up to the 108 Members of the Assembly to get into fresh negotiations to restore the necessary mutual trust between unionists, nationalists and republicans.
The decision to withdraw Danny Kennedy as Minister for Regional Development, probably from next week, will leave the department in the hands of civil servants.
While the pull-out still has to be endorsed by the UUP's ruling executive, it is to hold a special meeting this Saturday which is likely to rubber-stamp the move.
The withdrawal was backed yesterday by the party's MPs Tom Elliott and Danny Kinahan, MEP Jim Nicholson, members of its Assembly team and party officers.
Mr Nesbitt also said there had been no meetings with the DUP since the murder of Mr McGuigan and said it was up to Peter Robinson and his colleagues to make their own decisions.
"Since 2007, the DUP and Sinn Fein have been leading our Government. That's over eight years. We need wait no longer for further proof of their inability to deliver those goals."
But the DUP said the UUP record in power during former leader Lord David Trimble's years had been "crisis and collapse".
DUP deputy leader Nigel Dodds said: "The UUP previously sat in Government with Sinn Fein before decommissioning and whenever the PIRA was armed and active. For the UUP to try and rewrite history is downright hypocritical and misleading."
Sinn Fein immediately accused the UUP of attempting to manufacture a crisis. Deputy First Minister Martin McGuiness tweeted: "This decision by the UUP is more about inter Unionist rivalry than their & others feigned concern about our unequivocal commitment to #Peace."
His MLA colleague Gerry Kelly said: "It is very hard to believe what he is trying to do other than compete with the DUP in upcoming elections."
Alliance Executive minister Stephen Farry hit out: "It is clear the UUP does not have the best interests of everyone at heart, when they had no issue joining pan-unionist groupings which included representatives of loyalist paramilitary organisations."
But the move was predictably welcomed by TUV leader Jim Allister, who said: "If the DUP turns a blind eye then they are licensing the IRA to murder again. The time has come for the DUP and the Secretary of State to put an end to this corruption of democracy."