The Ulster Unionist Party leader has recommended his party withdraws from the North's power sharing Executive.
Mike Nesbitt says Sinn Féin has 'no credibility' - and the UUP has 'no trust' in the party.
In a statement, he added that the "decision is to withdraw from the Northern Ireland Executive, to form an Opposition and offer people an alternative, as is the way in any proper democracy.
"We are in a bad place but this can be fixed. But the IRA need to go away and stop terrorising their own communities."
Unionists had threatened to exclude Sinn Féin from Stormont after the PSNI said the Provisional IRA still exists - and linked its members to the murder of Kevin McGuigan in Belfast earlier this month.
The UUP will vote on the recommendation on Saturday.
The recommendation by the party, a minor partner in the five-party mandatory coalition, will not automatically cause the collapse of the administration. However it throws its future into serious doubt.
Pressure will now mount on the region's largest party, the Democratic Unionists, to do the same.
Sinn Féin Deputy First Minister Martin McGuinness has accused the UUP of playing party politics.
He tweeted:
This decision by the UUP is more about inter Unionist rivalry than their & others feigned concern about our unequivocal commitment to #Peace
— Martin McGuinness (@M_McGuinness_SF) August 26, 2015
In his weekly column in the Andersonstown News today, Gerry Adams said, "unionist politicians ignored the bits that didn’t fit with their narrative and jumped on Hamilton’s claim that the IRA still exists to ratchet up the crisis.
"There is no basis for the charges made against Sinn Féin by our political opponents and if this descends into a political crisis it is a direct result of their stupidity and party political opportunism," he added.
Chef constable George Hamilton said the Provisional IRA was not engaged in terrorism, and was instead pursuing peaceful, political republicanism.
However, he said that some of the paramilitary organisation's members were involved in the murder of Kevin McGuigan two weeks ago.
But Chief Constable Hamilton added there is no evidence the killing was sanctioned by the organisation.
Justice Minister Frances Fitzgerald has ordered gardaí to provide a "fresh assessment" of the activities of the Provisional IRA.
Northern Ireland Secretary Theresa Villiers said the UK government remains "fully committed" to implementing the Stormont House Agreement, which has been delayed by a fallout between Sinn Féin and the DUP.
