Senior members of the Ulster Unionist Party have voted to withdraw from Northern Ireland's power-sharing government over claims the Provisional IRA still exists.
UUP leader Mike Nesbitt proposed the exit in response to a police assessment that structures of the paramilitary organisation remain in place and some of its members were involved in a recent Belfast murder.
The UUP's ruling executive approved Mr Nesbitt's recommendation at a meeting in an east Belfast hotel.
Mr Nesbitt has said the revelations about the IRA have shattered trust in Sinn Fein and the UUP can no longer work in coalition with the republican party.
The vote involving around 90 members of the party's ruling body was unanimous in favour of a Stormont Executive walkout.
"The Ulster Unionist Party will be leaving the Northern Ireland Executive next week," Mr Nesbitt announced after the 90 minute meeting at the Park Avenue Hotel.
"This decision was unanimous."
Mr Nesbitt said Danny Kennedy, the UUP's one minister in a five-party administration comprising 13 ministers and two junior ministers, will formally resign next week.
The party said it will now form an "opposition" in the Assembly - even though the power-sharing structures do not afford recognition for an official opposition to the mandatory coalition government.
The exit by one of the three minor partners in the administration will not in itself trigger the collapse of the Executive but it will throw its future into serious doubt.
The UUP decision will heap pressure on the Democratic Unionists, one of the two major Executive partners, to follow suit - a move that would bring down power-sharing.
The DUP has so far insisted Sinn Féin should be the party leaving the Executive, not unionists. But the largest unionist party has made clear it will walk away if action is not taken to punish Sinn Féin.
The controversy was sparked after PSNI chief constable George Hamilton said the PIRA still exists and some members were involved in the murder earlier this month of Kevin McGuigan, 53, in co-operation with a group styling itself Action Against Drugs.
He has, however, said the PRIA is not engaged in terrorism and there is no evidence Mr McGuigan's murder was sanctioned by IRA leadership.
Mr Nesbitt claimed Sinn Féin's continued insistence that the IRA no longer exists means his party cannot trust what they say.
He said the UUP would now offer an "alternative" to the current Stormont government.
"Sinn Féin's position with regard to the murder of Mr McGuigan has broken all faith and trust," he said.
"Until that trust is restored and we find mechanisms to verify trust we cannot work in government with Sinn Féin."
Sinn Féin, which maintains the IRA has "gone away", has accused the UUP of contriving a crisis in a bid to outflank the DUP ahead of next year's Assembly poll.
