The Ulster Unionist Party intends to leave the Northern Ireland Executive, the party's leader has said.
Mike Nesbitt made the recommendation during a meeting with senior party members.
The announcement comes as part of an ongoing political row over the status of the Provisional IRA that followed the murder of Kevin McGuigan Sr.
Sinn Féin accused the UUP of creating a "crisis", while the DUP said the move was "hypocritical and misleading".
Mr Nesbitt said he had chaired a meeting attended by the party's MLAs, MEP, MPs, senior representatives of its councillors' association and its party chairman, and they had endorsed his recommendation "unanimously".
He said the party's ruling body would make a final decision on Saturday and if it was supported then the party would form an opposition.
The Northern Ireland Executive is a power-sharing government drawing ministers from the five biggest parties.
The Ulster Unionist announcement comes as part of a political row that has followed the Police Service of Northern Ireland's assertion that members of the Provisional IRA were involved in the murder of Mr McGuigan Sr.
The 53-year-old ex-IRA man, was killed in what police believe was part of a "fall-out" in republican circles after the murder of former IRA commander Gerard 'Jock' Davison in May.
Police said an infrastructure exists at a senior level of the Provisional IRA, but that there was no evidence that Mr McGuigan's murder was sanctioned by that hierarchy.
On Sunday, Sinn Féin president Gerry Adams said there was no reason for armed republican groups, such as the IRA, to exist as the movement was committed to peace.
Mr Nesbitt said Sinn Féin had "no credibility and we have no trust and without trust we have nothing".
"The situation can be fixed but we need some clarity about the IRA and its command structure," he said.
He said the DUP and Sinn Féin-led government had been "incapable of delivering positive outcomes".
Mr Nesbitt added that Danny Kennedy, the UUP minister for regional development, would resign if the recommendation was endorsed.
Analysis: Chris Page, Political Correspondent
It is very likely that the UUP's ruling body will endorse the plan to withdraw from the executive, given that the party's senior elected representatives are unanimously backing it.
The Ulster Unionists have just one minister - Danny Kennedy - out of 13 around the Executive table.
So the executive will continue to function without the UUP.
But the Ulster Unionists' move will put pressure on the Democratic Unionist Party, the biggest party in the assembly, to take action.
The party executive is expected to make a decision on Saturday.
"We will then form an opposition and offer the voter an alternative as is normal in any democracy," Mr Nesbitt added.
"It has not been an easy decision but we believe it is the right thing to do at this time."
The DUP 's deputy leader Nigel Dodds said the UUP's record in government "is one of crisis and collapse".
"The UUP previously sat in government with Sinn Féin before decommissioning and whenever the Provisional IRA was armed and active.
"For the UUP to try and rewrite history is downright hypocritical and misleading."
He added: "If anyone should be excluded from government in Northern Ireland for wrongdoing it is Sinn Féin, not unionists."
Sinn Féin's Gerry Kelly accused Mr Nesbitt of electioneering and trying to "create a crisis". He reiterated his party's comments that the "IRA has left the stage".
"This was a party political broadcast as opposed to trying to resolve situations here," he said.
"This is based in hypocrisy. Last year, he [Mike Nesbitt] walked out with his party and went shoulder-to-shoulder with people connected to paramilitary organisations.
"It is very hard to believe what he is trying to do other than compete with the DUP in upcoming elections.
"This is old hat, there is nothing new in any of this, he believes that he is making a good move towards the elections. It is electioneering."
The Northern Ireland Secretary of State Theresa Villiers said the announcement "was a matter for the Ulster Unionist Party who take their own decisions".
"The government remains fully committed to the devolved political institutions and to the implementation of the Stormont House Agreement," she added.
"Over the coming days, I shall be continuing my discussions with the parties about fallout from the murder of Kevin McGuigan."