Crunch talks on Northern Ireland's deteriorating political situation will take place in London and Dublin.
David Cameron is expected to come under pressure to penalise Sinn Fein when he meets Peter Robinson and his Democratic Unionist Party deputy Nigel Dodds at Downing Street today.
The political fallout following the murder of IRA man Kevin McGuigan will also top the agenda at Iveagh House where Northern Ireland Secretary Theresa Villiers will sit down with Irish Foreign Affairs Minister Charlie Flanagan and Justice Minister Frances Fitzgerald who have tasked Garda chiefs with reviewing Provisional IRA (PIRA) activity.
The controversy was sparked when Police Service of Northern Ireland (PSNI) chief constable George Hamilton said the PIRA still exists and some members, along with a group styling itself Action Against Drugs, were involved in the murder of the 53-year-old father-of-nine earlier this month.
Police believe the killing was a revenge attack by republican associates of IRA commander Gerard 'Jock' Davison who was gunned down in May.
The chief constable 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.
But the Ulster Unionist Party (UUP) said it could no longer work with Sinn Fein because trust has been shattered.
On Saturday it unanimously voted to leave the power-sharing Executive and the party's only minister Danny Kennedy is due to resign today.
Although the move is not enough to collapse the Executive it has put pressure on the DUP - the largest unionist party in the Assembly.
Peter Robinson, who has been on holiday, branded the UUP decision irrational, illogical and based on "political expediency" rather than principle.
Walking away should be a last resort, he said.
Meanwhile, Sinn Fein said it would not be "deflected" and accused political opponents of exploiting murder.
Conor Murphy, MLA for Newry and Armagh, said: "We and the 178,000 people who voted for us in the last Assembly elections will not be excluded or discriminated against. Those days are over."