Sinn Féin blocks welfare bill in Northern Ireland Assembly
The Northern Ireland Assembly is in crisis after Sinn Féin suddenly withdrew its support for welfare reform.
Sinn Féin and the Democratic Unionist Party, partners in the executive, have accused each other of bad faith.
Moving to block a welfare reform bill, Martin McGuinness, of Sinn Féin, accused the DUP of reneging on commitments.
DUP leader Peter Robinson said that was "dishonourable and ham-fisted".
A spokesman for David Cameron said the prime minister found the development "deeply concerning".
He said it was "very important" that the Stormont House Agreement, which included a deal on welfare reform, was implemented in full, and that "it must be for the locally elected political leadership to find a way forward, the responsibility is with them""
Welfare reform was the issue that threatened the future of power-sharing at Stormont last year.
The five main parties reached broad agreement on 23 December on a number of key issues, including welfare.
It followed 12 weeks of talks involving the Northern Ireland parties and the British and Irish governments.
However, Mr McGuinness, Northern Ireland's deputy first minister, said the DUP had reneged on its commitments in the Stormont House Agreement to protect the most vulnerable people in society.
He described it as a "very serious situation" that could have "profound implications" for both the agreement and the Northern Ireland political institutions.
"At Stormont House, the five parties agreed a series of measures to protect the vulnerable and safeguard current and future welfare claimants under the control of the executive," he said.
"However, the DUP have acted in bad faith and are now reneging on their commitments to protect the most vulnerable. It is their intention to provide only partial protection to current recipients of benefit and no protection whatsoever for future claimants. That is totally unacceptable."
Sinn Féin president Gerry Adams said on Monday night that until the DUP social development minister produces a scheme, "which reflects what was agreed at Christmas" his party would not support the welfare bill.
"Of course, we are very mindful that our opposition to the welfare bill could have wider implications for the political institutions and the Stormont House Agreement but there can be no reneging on this issue," he said.
"Sinn Féin is equally committed to finding an acceptable resolution on the terms agreed at the Stormont House Agreement."
Analysis: Gareth Gordon, BBC NI political correspondent
As every trainee journalist knows there are five basic questions that must be answered before you do any story: Who?; What?; Why?; Where? and When?
The Who? with this story is easy - that's Sinn Féin.
The What? is that they have essentially pulled the rug from beneath the Welfare Reform Bill, the one everybody thought was sorted before this morning.
The difficult one is the Why?
Read more here
Northern Ireland First Minister Peter Robinson said: "I've been involved now in politics in Northern Ireland for 47 years and I have to say that in all of that time I have never seen such a dishonourable, ham-fisted statement as the one issued by Sinn Féin today.
"Most people are scratching their heads wondering what on earth has Martin McGuinness been taking over the last number of days.
"The bad faith will be on the part of anybody who refuses to stand up for what they agreed at Stormont Castle and at Stormont House.
"I was completely dumbfounded as to how anybody could credibly make that kind of statement when they knew that there was a document in the public arena which showed precisely what had been agreed."
SDLP leader Alasdair McDonnell welcomed what he said was a U-turn by Sinn Féin.
"Martin McGuinness stood at his ard fheis [party conference] on Friday and championed the protections that this bill had put in place. What has changed in 48 hours?" he asked.
"Sinn Féin have recklessly and blindly followed the DUP without securing firm guarantees. Our amendments were a way of doing that and I'm glad today that Sinn Féin have woken up to the reality and joined us in our principled opposition to this attack on the vulnerable."
Northern Ireland Secretary Theresa Villiers said failure to make progress on the issue of welfare would have serious implications for the executive.
"The recently agreed 2015-16 budget will not be deliverable in its current form," she said.
Ivan Lewis, Shadow Northern Ireland Secretary, said Sinn Féin's decision to withdraw their support for the welfare reform bill was "a very serious development which threatens political stability in Northern Ireland".
Petition of concern
Sinn Féin lodged a petition of concern to stop the bill's passage in the assembly.
If a petition of concern is presented to the assembly speaker any vote taken by MLAs will have to have support from both unionist and nationalists.
A proposed legislation or motions will then only pass if supported by a weighted majority (60%) of members voting, including at least 40% of each of the nationalist and unionist designations present and voting.
Effectively this means that, provided enough MLAs from a given community agree, that community can exercise a veto over the assembly's decisions.
Earlier, Ulster Unionist leader Mike Nesbitt was asked how he would react if the SDLP signed Sinn Féin's petition of concern to block the welfare reform bill.
Mr Nesbitt replied: "I wouldn't be particularly surprised because they're under extreme political pressure so to do, but if they do it, they know what they're doing.
"They are bringing, potentially, the entire house down."
Sinn Féin and the SDLP had opposed welfare reform plans agreed at Westminster, but it had been thought that the issue had been resolved by the Stormont House Agreement.
In agreeing to move forward on welfare reform, the parties in Northern Ireland agreed to introduce a number of new schemes to ensure additional financial support was directed to those set to lose out by changes to the benefits system.
Stewart Dickson of the Alliance Party said Sinn Féin now had "egg on their face".
"It is grossly disingenuous of Sinn Fein to say they didn't know what was going to happen and that they didn't know what these figures actually meant," he said.
"What planet is Sinn Féin on? From 6 April we face a further £114m fine from Westminster. The whole situation is now in crisis today.
"They knew exactly what they were signing up to during the talks."