Robinson said on Thursday evening, shortly after rival parties rejected his proposal to suspend the Stormont parliament in Northern Ireland, that he would uphold his earlier threat and quit government. Robinson's Democratic Unionist Party (DUP), which favors continued UK membership, is the largest in the assembly.

"In light of the decision 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," Robinson told reporters after crisis talks in Belfast. "I have asked Arlene to remain in her 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."

The British government in Westminster could still unilaterally decide to suspend parliament, Robinson said. However, Northern Ireland Secretary Theresa Villiers had said earlier this week, when Robinson's resignation was only an unfulfilled threat, that the UK government "does not feel that the time would be right to suspend the institutions at this stage and under these circumstances."

An initial statement from David Cameron's spokeswoman on Thursday evening said only that the British prime minister was "gravely concerned" by developments, and that "we want to see all politicians in Northern Ireland working together to build a better future for the country."

Robinson also said on Thursday that multi-party talks in Northern Ireland would continue despite his resignation as first minister.

How it came to this

The murder in August of Kevin McGuigan, an alleged former Irish Republican Army (IRA) hit man, helped bring Northern Ireland's power sharing government to the brink of collapse. The police said that it suspected the IRA in the killing - a move that put nationalist party Sinn Fein under pressure. Several high profile arrests followed, not least that of senior Sinn Fein figure Bobby Storey in Belfast on Wednesday. He was released the next day.

During the roughly 30 years of sectarian violence known as The Troubles, Sinn Fein was the political wing of the IRA.

Robinson and the DUP alleged that the killing showed both that the IRA existed, and was still active and armed, in breach of the 1998 Good Friday Agreement that underpins the power-sharing agreement at Stormont.

"One of the ironies of this is that a lot of unionists would have absolutely no sympathy for someone like Kevin McGuigan; he was known to be an IRA assasin, he had killed many people. And yet, as a result of his murder, they are the ones who are potentially pulling Stormont down - in protest at the fact that he has been killed," Sam McBride, political correspondent for the "News Letter" daily in Northern Ireland, told DW.

Sinn Fein - a predominantly Catholic party advocating that Northern Ireland leave the UK to join with Ireland - maintains that the IRA is no longer active. Its leader, Northern Ireland's Deputy First Minister, Martin McGuinness, said he believed Robinson's plans to quit were down to "inter-party rivalry" between the DUP and fellow pro-UK party the Ulster Unionist Party, which itself resigned from the government on Monday. Suspending the parliament, he said ahead of Thursday's vote, would be a mistake.

"I think it would send a very negative message and would be grist to the mill of those who in the past have tried to plunge us back to the past," McGuinness said.

A grand coalition, plus a tough past

In some sense, Northern Ireland's power-sharing deal rather mirrors Germany's current "grand coalition" - the two largest parties and main rivals are ruling together, while there's practically no opposition in Germany, and none at all in Northern Ireland. Sinn Fein leans to the left, and has become a strong voice against austerity north and south of Ireland's border; the DUP, meanwhile, is seen as a conservative player on the political landscape.

But while Angela Merkel and Sigmar Gabriel might butt heads on policy within Germany's coalition, Robinson and McGuinness had to keep the rainbow coalition running day-to-day despite the deeper ideological differences they hold over Northern Ireland's past and future.

The last time Stormont was suspended, in 2002, it took almost five years to tempt the opposing sides back into unity government.

"I think the catalyst for the current problem has been the murder of Kevin McGuigan about a month ago, but really that was the straw that broke the camel's back rather than the source of the problem. It's true that probably the entire Stormont that we've had since 2007 has been fairly unstable, in the sense that it's always one crisis away from collapsing," McBride says.

Budget cuts, and where to implement them, had been a particular sticking point in recent years, McBride explained, not least amidst a heated UK election campaign earlier this year, and with Northern Irish parliamentary elections scheduled for May next year.