Power-sharing in Northern Ireland stands on the brink after First Minister Peter Robinson stepped down and the majority of his Executive ministers resigned amid a crisis sparked by a murder linked to the IRA.
Democratic Unionist leader Mr Robinson has asked current DUP finance minister Arlene Foster to remain in the Executive and take over as acting First Minister. Prime Minister David Cameron was tonight said to be "gravely concerned" about the situation at Stormont.
The mass walkout from the mandatory coalition came after the DUP failed to get the Assembly adjourned for a period to allow crisis talks to address the implications of the murder of Kevin McGuigan to take place.
The political furore over the killing intensified on Wednesday when three senior republicans, including Sinn Fein's northern chairman Bobby Storey, were arrested in connection with the murder of Kevin McGuigan. However, Mr Storey and another of the arrested men were released from custody without charge tonight.
As he announced the resignations, Mr Robinson repeated a demand for the Government to suspend the institutions outright to enable space for the talks to happen. Northern Ireland Secretary Theresa Villiers rejected the call.
The fallout from the murder of Kevin McGuigan has already seen the Ulster Unionists resign their one ministerial post. The exit of Mr Robinson along with three of the DUP's four other ministers, and its one junior minister, has left the 13 minister administration in freefall. The departments of health and social care; social development; enterprise trade and investment; and regional development are now effectively rudderless.
Collapse of power-sharing is not inevitable but its demise appears to have been hastened by a day of dramatic developments at Stormont.
The DUP wanted all Assembly business suspended to allow crisis talks to take place about the political consequences of the murder of Mr McGuigan.
Mr Robinson's announcement came after Sinn Fein, the SDLP and the Ulster Unionists voted against a DUP proposal to adjourn the Assembly.
He issued a resignation ultimatum on Wednesday after the arrest of republicans Mr Storey, Eddie Copeland and Brian Gillen over the fatal shooting of former IRA man Mr McGuigan. Mr Copeland and Mr Storey were released this evening.
Police have said current members of the IRA were involved in last month's shooting of Mr McGuigan in a suspected revenge attack for the murder of former IRA commander Gerard "Jock" Davison in Belfast three months earlier.
The revelations about the IRA have heaped pressure on Sinn Fein to explain why the supposedly defunct paramilitary organisation is still in existence.
Mrs Foster, a Fermanagh and South Tyrone MLA, is the current finance minister and she will also continue in that role.
Mr Robinson said: "In light of the decision by republicans, nationalists and the UUP to continue with business as usual in the Assembly, I am therefore standing aside as First Minister and other DUP ministers will resign with immediate effect with the exception of Arlene Foster.
"I have asked Arlene to remain in post as Finance Minister and acting First Minister to ensure that nationalists and republicans are not able to take financial and other decisions that may be detrimental to Northern Ireland."
Ms Villiers said she would not be suspending the devolved institutions and called on the local parties to come together.
She said the DUP resignations would mean the functioning of the Executive became much more difficult.
"It is a sign of a complete breakdown in working relationships within the Executive," she said.
In the wake of his release, Mr Storey's lawyer said his client intended to sue for wrongful arrest.
Sinn Fein has insisted the IRA has gone away and has accused both unionist parties of ramping up the crisis for electoral gain.
Sinn Fein president Gerry Adams, welcoming the Assembly business committee decision to reject an adjournment, said: "The decision of the business committee is a very, very clear democratic reiteration of the integrity of these institutions and of the need and the wish for these institutions to continue the work which we were all elected to do on behalf of citizens in this state and across this island."
Irish premier Taoiseach Enda Kenny had urged the SDLP not to vote against adjournment in an eleventh hour meeting in Dublin this morning. However, the nationalist party, which was one of the architects of the 1998 Good Friday Agreement, ultimately decided to oppose the move.
Ulster Unionist leader Mike Nesbitt has stated that the Northern Ireland Executive needs surgery rather than a sticking plaster.
"This is about a murder," he said. "It is about the status of a terrorist organisation in 2015. And it is about the denial of that by a party of government - Sinn Fein. That's why we withdrew from the Executive. We were not prepared to turn a blind eye."
Alliance leader David Ford heavily criticised the UUP and SDLP and accused them of betraying the past generations of their parties which helped forge the 1998 Good Friday Agreement.
Charlie Flanagan, Ireland's Foreign Affairs Minister, said he greatly regretted the DUP's mass resignations.
"What we want is for the devolved power-sharing Executive and Assembly to work as envisaged in the Good Friday Agreement and deliver peace, prosperity and reconciliation for the people of Northern Ireland," he said.
A Downing Street spokesman said; "The Prime Minister is gravely concerned about the situation in Northern Ireland following developments there today.
"Earlier this afternoon, the Prime Minister spoke with Peter Robinson. While acknowledging the gravity of the situation, the Prime Minister told Mr Robinson that the UK Government did not believe it would be right to introduce emergency legislation now to suspend the Assembly.
"They discussed options for what more the UK Government could do to comprehensively address all remaining paramilitary activity in Northern Ireland.
"The PM underlined the need for intensive cross-party talks to identify ways to tackle all paramilitary groups and to get on with implementation of the Stormont House Agreement
"The Prime Minister said that there should be a return to the spirit that had seen politicians show such leadership over the years to deliver a peace process that has inspired people across the world.
"The Government objective is clear: we want to work with political leaders for a Northern Ireland where politics works, the economy grows and which is no longer defined by its divided past but by its shared future."
Mr Gillen was later also released from custody.
The US administration urged Northern Ireland's leaders to resolve the outstanding problems.
Secretary of State John Kerry's envoy to Northern Ireland, former senator Gary Hart said: "The United States Government strongly supports the cross-party talks led by the UK and Irish governments.
"We again encourage all of Northern Ireland's political parties to engage constructively in these negotiations in order to strengthen the devolved institutions.
"Northern Ireland's achievements are the result of dialogue and responsible leadership; now is the time to re-energise the parties' commitment to implementing the Stormont House Agreement and addressing remnant paramilitary activity.
"The United States remains actively engaged and will continue to support political progress."