The Ulster Unionist Party's sole minister Danny Kennedy is to formally resign on Tuesday following his party's withdrawal from the Executive.
Senior members of the UUP voted to withdraw from Northern Ireland's power-sharing government at the weekend over claims the Provisional IRA were involved in the murder of Kevin McGuigan.
Party leader Mike Nesbitt proposed the move in response to the PSNI assessment that structures of the IRA remain in place and some of its members were involved in the east Belfast murder.
The UUP's ruling executive formally approved the recommendation on Saturday with all 90 members voting unanimously to walk away from the Stormont executive. Mr Nesbitt has said the revelations about the IRA have "shattered trust" in Sinn Féin and the UUP can no longer work in coalition with the republican party.
"The Ulster Unionist Party will be leaving the Northern Ireland Executive next week," Mr Nesbitt said.
"This decision was unanimous.
"Sinn Féin's position with regard to the murder of Mr McGuigan has broken all faith and trust," he said.
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"Until that trust is restored and we find mechanisms to verify trust we cannot work in government with Sinn Féin", he added.
Mr Nesbitt said Danny Kennedy, the UUP's one minister in a five-party administration will formally resign on Tuesday.
The party is expected to now form an "opposition" in the Assembly - even though the power-sharing structures have no current facility for an official opposition to the mandatory coalition government.
Pressure is now building on the DUP to follow suit - a move that would collapse the power-sharing government and force a period of direct rule from Westminster .
To date the DUP have insisted Sinn Féin should be forced from the Executive, but the largest unionist party has said it will walk away if action is not taken to punish republicans.
Chief constable George Hamilton previously said the IRA still exists and some members were involved in the murder of the father of nine on August 12, saying they were acting in a joint enterprise with vigilante group Action Against Drugs.
Mr Hamilton however, added that the PIRA is not engaged in terrorism and that there is no evidence the McGuigan killing was sanctioned by the organisation's leadership.
Mr McGuigan was suspected of involvement in the murder of former IRA leader Gerard "Jock" Davison in Belfast three months ago. Police believe his killing was a revenge attack by the IRA.
Sinn Féin, have accused the UUP of contriving a political crisis in a bid to outflank the DUP ahead of next year's Assembly elections and branded the party hypocrites for sitting on the unionist forum last year with leading loyalist figures linked to both the UVF and UDA.
Meanwhile Irish Foreign Affairs Minister Charlie Flanagan has described the political situation at Stormont as "grave" adding that there was no benefit in "car crash politics".
Speaking ahead of a planned meeting with Secretary of State Teresa Villiers this week Mr Flanagan said people did not believe Sinn Féin when they denied the existence of the IRA and that instead the party would be better served to use influence to help disband the organisation.
"I believe that normal politics will only be introduced on this island fully if Sinn Féin uses its influence to ensure that the IRA is put firmly out of business", Mr Flanagan added.
SDLP MLA Alex Attwood criticised the UUP decision to withdraw but said Sinn Féin also had a responsibility for the failing institutions.
"Republicans too have failed the (Good Friday) Agreement. Breaches on decommissioning, holding back on policing and now on the NCA (National Crime Agency), the murders of Paul Quinn and Robert McCartney, repeated denials by Sinn Féin of the truth of the IRA's past show how it is not only some in political unionism who cling to the past. All of this has eroded the confidence of pro-agreement unionism. Current denials by Sinn Féin only erode it more."
