The Democratic Unionists have warned they are prepared to pull the plug on Northern Ireland's power-sharing government over claims the Provisional IRA still exists.
DUP MP Jeffrey Donaldson said his party would not hesitate to act unilaterally if others did not support any future move to exclude Sinn Fein from the coalition administration.
A DUP delegation will meet Northern Ireland Secretary Theresa Villiers in Belfast later to discuss the furore sparked by a recent murder in the city and a police assessment that Provisional IRA members were involved.
The DUP's main electoral rivals, the Ulster Unionists, are set to resign from the Stormont Executive next week over the revelations.
While the dramatic walkout by one of the three minor coalition partners will not automatically trigger the collapse of the administration, it does throw its future into serious doubt, as it mounts pressure on the DUP to follow suit.
If the DUP - Northern Ireland's largest party - left the Executive, it would fold immediately.
A bid by the DUP to exclude Sinn Fein from the administration would need the support of Ms Villiers and other Executive parties.
But in the absence of obtaining that backing, the DUP could bring down the institutions by resigning its First Minister post held by Peter Robinson.
Mr Donaldson said the only people punished by the UUP move were its voters
"They are excluded from the government, not Sinn Fein," he said.
"It is only the DUP who can now deal with this issue, sadly."
'Uphold the rule of law'
Mr Donaldson told Radio 4's Today Programme: "We are going to move to exclude Sinn Fein from the government. We believe the people who should be punished here are the people who have done wrong, not the people who have stood by their pledges, who uphold the rule of law and who support democracy.
"In the end, if the other parties are not prepared to support the exclusion of Sinn Fein then we will act unilaterally and if that means that we have a period in Northern Ireland where we don't have a government until we resolve and sort out these issues then so be it."
Announcing the UUP walkout on Wednesday, party leader Mike Nesbitt claimed trust in Sinn Fein had been shattered and his party has no option other than to withdraw from the coalition and form an opposition in the Assembly.
Sinn Fein has accused the UUP of contriving a crisis in a bid to outflank the DUP ahead of next year's Assembly poll.
At the weekend, Police Service of Northern Ireland (PSNI) Chief Constable George Hamilton said that the IRA still exists, but is not engaged in terrorism - instead pursuing peaceful, political republicanism.
But the PSNI also said some Provisional IRA members were involved in the murder earlier this month of former IRA man Kevin McGuigan, 53, in co-operation with a group styling itself Action Against Drugs. Detectives said there is no evidence the killing was sanctioned by the IRA leadership.
Mr McGuigan was suspected by some in the republican movement 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 Mr Davison's republican associates.
It is almost 20 years since the Provisional IRA's last ceasefire and a decade on from the supposed decommissioning of its weapons.
The UUP's one minister in an administration made up of 13 ministers and two junior ministers will resign next week if Mr Nesbitt's recommendation is supported by the party's ruling executive on Saturday - an endorsement that is widely expected.
An Ulster Unionist exit from the Executive would be highly symbolic given the party was one of the architects of the 1998 Good Friday peace agreement that paved the way for nationalists and unionists to share power.
The accord did not envisage an Assembly with an official opposition.