Northern Ireland's First Minister is to meet the Prime Minister tomorrow to discuss Stormont's deepening political crisis.
Peter Robinson and his Democratic Unionist Party (DUP) deputy leader Nigel Dodds are expected to travel to Downing Street for crunch talks with David Cameron.
It comes after the Ulster Unionist Party (UUP) voted to withdraw from the power-sharing Executive in the wake of revelations that the Provisional IRA still exists.
Mr Robinson, who has been on holiday, branded the UUP decision irrational, illogical and based on "political expediency" rather than principle.
He said walking away should be a last resort.
"This is not the time to flee the battlefield, it is the time to confront violent republicanism, to stand and fight for democratic principles and to do what is right for the law abiding citizens of Northern Ireland," said Mr Robinson.
The latest furore was sparked after Police Service of Northern Ireland (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.
Mr Hamilton has, however, said the PIRA is not engaged in terrorism - instead pursuing peaceful, political republicanism - and that there is no evidence the McGuigan 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.
Mr Robinson said he accepted a police assertion that IRA members were involved in the murder of Mr McGuigan and that some IRA structures remain in place.
Such activity is inconsistent with membership of the Executive, he said.
Writing in the Belfast Telegraph, Mr Robinson added: "We must not allow republicans who precipitated the crisis, to end up benefiting from it. It should be republicans and not unionists who pay the price for the misbehaviour of the IRA.
UUP leader Mike Nesbitt said his party was unable to work with Sinn Fein because the revelations about the IRA had shattered trust.
His recommendation to pull out of the five-party Executive was unanimously backed by 90 members of the UUP's governing body during a vote on Saturday.
The party's only minister Danny Kennedy will formally resign from the Executive tomorrow.
In a hard-hitting statement, Mr Nesbitt atacked the DUP for not following the Ulster Unionists out of the Executive but said he would not be "deflected".
He said: " It is extraordinary that it has taken until the last day of August for Peter Robinson to comment on the latest IRA murder. It is equally remarkable that he cannot agree with me that enough is enough and take a principled stand."