Deputy first minister Martin McGuinness has said the Northern Ireland assembly is facing a "very serious crisis" over welfare reform.
It comes as Sinn Fein blocked a welfare reform bill as Mr McGuinness claimed the DUP had acted in bad faith on welfare protections.
He said the DUP had reneged on its commitments in the Stormont House Agreement to protect the most vulnerable people in society.
Sinn Fein then submitted a petition of concern.
First minister Peter Robinson has slammed Sinn Fein's announcement that they are to oppose the passage of the welfare bill in the Northern Ireland assembly as "dishonourable and ham-fisted".
The DUP leader said he will "implement every word and number in the Stormont House and Castle agreements".
The five main parties reached broad agreement on December 23 on a range of key issues -including welfare reform- which has been at the centre of a political wrangle.
It followed 100 hours of talks involving the Northern Ireland parties and two governments - Prime Minister David Cameron and Taoiseach Enda Kenny took part in an intensive session of talks in Belfast.
Sinn Fein had since supported the legislation through the Assembly but, ahead of today's final stage in the chamber, the party reversed its stance.
The Stormont House Agreement essentially paved the way for the implementation of the Government's welfare reforms alongside a series of Stormont-funded packages to support those whose benefits were set to be cut.
Secretary of State Theresa Villiers said the developments were "deeply concerning".
Mr McGuinness said: “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.
"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.
Another Stormont tug-of-war and tug-of-words. Welfare fallout.— Brian Rowan (@BrianPJRowan) March 9, 2015
“That is totally unacceptable.
"If the DUP want to strip benefits from children with disabilities, from adults with severe disabilities, the long-term sick; or push children further into poverty, then they need to explain and justify that. Sinn Féin certainly will not accept that approach.
“Until such times as the minister can produce a scheme for agreement which gives effect to the intent of the Stormont House Agreement by providing full protection for current and future claimants, Sinn Féin will not be in a position to support the Welfare Bill going through the Assembly.
“The DUP have attempted to effect Tory welfare cuts by subterfuge but at the heart of this crisis is the ideologically driven attack on the welfare state by the Tory-led government in London.
“As we have repeatedly stated publicly Sinn Féin will not be part of any agenda that punishes the most vulnerable in our society.”
Mr McGuinness added: "This represents a very serious crisis. It is a crisis that needs to be averted. We need to see a resolution of that crisis."
Mr Robinson said nobody could have been in any doubt about the coverage of the additional entitlements to benefits decided at Stormont House.
He said extra money from the Stormont House Agreement was never going to cover the additional sums needed for welfare following cuts imposed by the Coalition at Westminster.
"This conundrum was always the same for every party in the Executive.
"We had an agreement, we put the figures down so that there was no doubt, nobody can be in any doubt what we agreed to."
He added: "I have never seen such a dishonourable, ham-fisted statement as the one issued by Sinn Fein today.
"The DUP will implement every word and number in the Stormont House and Stormont Castle Agreements."
The Assembly was due to legislate on welfare reform today. As part of the deal powers over corporation tax were due to be devolved from London to Belfast, allowing Northern Ireland to reduce the levy on business profits to compete for investment with the Republic of Ireland.
Mr Robinson said: "The consequences will be that the (Westminster) Corporation Tax Bill will not go forward. If this bill is defeated in the Assembly today it will have implications for our budget, the money to deal with restructuring of the public service will no longer be available to us.
"The Stormont House Agreement would fall because this is a key element of it.
"If the Stormont House Agreement falls then we are back into a crisis situation."
He added: "They (Sinn Fein) have decided to bring the show to an end."
Secretary of State Theresa Villiers urged the parties to work together to resolve the issue.
"Changes to the welfare system in Northern Ireland were a key part of the Stormont House Agreement and without these changes it will be extremely difficult for the Executive to deliver a balanced budget", she said.
"It remains pivotal that all aspects of the Agreement are implemented in full. Failure to make progress on the issue of welfare has serious implications for the Executive. The recently agreed 2015-16 budget will not be deliverable in its current form."
"I urge the Northern Ireland parties to work together to resolve this issue and continue the progress made so far in implementing the Stormont House Agreement."
"The Agreement offers us the prospects of stability, and of growing prosperity, building on Corporation Tax devolution. We must not put those prospects in peril. The situation requires everyone to show leadership and responsibility."
SDLP leader Dr Alasdair McDonnell welcomed Sinn Fein's "U-Turn" on welfare reform and have signed the petition of concern.
The party tweeted: "We have stood strong on Welfare when others were weak. We will continue to defend the struggling. Petition signed."
He said: “Make no mistake about it, this is a U-Turn and Sinn Féin, having abused us for asking questions, are now backing the position that we have held from the very beginning.
“The SDLP has led the opposition to these aspects of Welfare reform that punish the struggling, the sick and the vulnerable. Throughout this fight we have been frustrated by the ignorance of the DUP and Sinn Féin who vetoed, voted against and disrupted our reasoned attempt to salvage this legislation.
“Martin McGuinness stood at his Ard Fheis on Friday and championed the protections that this bill had put in place. What has changed in 48 hours?
“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.”
Alliance Leader David Ford criticised Sinn Fein's decision - which he says has threatened the Stormont House Agreement.
Mr Ford said: “We have now seen an unravelling of the Stormont House Agreement which sought to move our economy forward and resolve issues that were holding our society back such as dealing with the past.
“Sinn Fein is either being utterly cynical or utterly cowardly, or even utterly stupid. Everybody was fully aware that the welfare arrangements were only temporary transitional measures. It is simply not credible for Sinn Fein to suggest otherwise.
“The petition of concern that has been submitted by Sinn Fein, the SDLP and the Green Party has put the institutions at risk. This is not about protecting the most vulnerable in our society; it is about parties protecting themselves.
"They are damaging our chances of mitigating the worst aspects of the welfare changes and of protecting our public services.
“It appears that Sinn Fein are doing this for electioneering purposes. They claim to be opposed to austerity in the South and are frightened to be seen to be implementing it in the North.”
The introduction of welfare reforms in Northern Ireland have been delayed by over two years, primarily due to Sinn Fein's reluctance to sign up to measures it claimed would hit the most vulnerable.
The row appeared to have been sorted in December's Stormont House political deal when the five Executive parties agreed to offer additional financial assistance to claimants from its own pockets.
Northern Ireland's leaders had warned that the very future of the power-sharing Executive would have been under threat had the agreement not been reached - as multi-million pound Treasury penalties for non-implementation would have been too much to shoulder.
Sinn Fein's announcement casts doubt over the progress supposedly achieved in December.
Essentially the same welfare reforms that proved contentious in Great Britain were to be brought into Northern Ireland - among them the so-called bedroom tax; the £26,000 cap on benefit claims; Universal Credit; and the replacement of Disability Living Allowance with Personal Independence Payment.
But the key difference was that parties in Northern Ireland had 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.
So while Northern Ireland would have maintained parity with the rest of the UK in terms of the relative cost to the Treasury, claimants in the region were set to benefit from a more generous system due to the added contribution direct from the Executive's coffers.
The parties had agreed to provisionally set aside around £560 million over the next six years to provide top-up payments to thousands of claimants.
Sinn Fein now claims the DUP has gone against its word in regard to how those Executive-funded measures would work.
The Treasury had been imposing multimillion-pound penalties on the Executive for its long-standing failure to implement the reforms.
In total around £100 million has already been taken off the block grant in the last two years - to reflect the region's failure to save the Treasury what has been saved elsewhere in the UK - and £114 million will be cut in the coming financial year if the changes are not implemented.