Peter Robinson has resigned as Northern Ireland's First Minister as the power-sharing agreement came close to collapse.
The Democratic Unionist leader carried out his threat to quit, along with all but one of the party's ministers in the devolved government, after opposition parties rejected its proposal to adjourn the Northern Ireland Assembly following allegations that Sinn Fein members were linked to a murder of a former IRA man last month.
DUP finance minister Arlene Foster will remain in government, taking over as temporary First Minister to prevent republicans taking control of the executive's finances.
Downing Street said David Cameron was "gravely concerned" about the deepening crisis at Stormont.
Earlier Mr Robinson said his DUP ministers would quit the Northern Ireland government on if the Assembly was not adjourned or suspended by the British government, which would force the collapse of Stormont’s power-sharing agreement.
The DUP has sought adjournment or suspension of the Assembly while cross-party talks on the paramilitary crisis are completed. But its proposal was opposed by the Ulster Unionists, the SDLP and Sinn Féin.
The fall of the devolved government has been at the centre of the peace agreement in Northern Ireland and its collapse would be unprecedented, risking the reopening of bitter divisions in the region.
The onus now switches to Mr Cameron, who is holding talks with political leaders and considering the whether to take the significant step of suspending the Northern Ireland Assembly.
The political row erupted after police announced they had arrested three prominent republicans in connection to the murder of Kevin McGuigan Sr, a former member of the Provisional IRA, in Belfast last month and allegations that the IRA still existed.
Sinn Fein’s Northern chairman Bobby Storey, Eddie Copeland and Brian Gillen were questioned at the Serious Crime Suite in Antrim.
Sinn Fein’s deputy first minister Martin McGuinness said it would be a “grave mistake” by the British government to suspend the political institutions in Northern Ireland “at the behest of the Unionist parties”. He accused the DUP of a failure of leadership.
Mr Cameron's official spokeswoman said: "The Prime Minister is gravely concerned about the situation. As he was saying in the House yesterday, we want to see all politicians in Northern Ireland working together to build a better future for the country and working to fulfil its great potential.We have been encouraging talks between the parties so they can work through their issues."
Asked whether the PM was considering suspending the Assembly, the spokeswoman said: "There are obviously now different people calling for different things, and the Prime Minister's calls with the Secretary of State and the First Minister are an opportunity for us to consider what steps should be taken next."Reuse content