Northern Ireland's devolved government is facing suspension or possible collapse amid a row over the budget.
The crisis has been sparked by the failure of the assembly to pass a welfare reform bill agreed in Westminster in 2013.
The nationalist parties at Stormont introduced a petition of concern, essentially a veto, which requires any bill to have sufficient support from both unionist and nationalist parties to go through.
Sky's Ireland Correspondent David Blevins said this means the devolved government is in "very real trouble".
He explained: "In short, the devolved government never implemented the welfare cuts agreed in Westminster in 2013.
"The parties finally reached agreement on a way forward last Christmas, the Stormont House agreement.
"But Sinn Fein had a change of heart in March of this year and withdrew its support for the bill.
"The problem is that Sinn Fein finds itself in a difficult position. It cannot be anti-austerity in the Republic of Ireland but push through an austerity budget in Northern Ireland."
Blevins added: "David Cameron will now have to decide whether he triggers a suspension for the first time in eight years or allows the devolved government to fall and move to an election."
