The First Minister and deputy First Minister fly out to the United States today on an economic mission, leaving behind a deepening crisis over Sinn Féin’s shock withdrawal of support for the welfare reform package hammered out at Stormont House.

Yesterday Sinn Féin accused the DUP of reneging on commitments made at the December talks on the future protection of welfare recipients. Sinn Féin succeeded in amassing the signatures of the 30 MLAs required to raise a petition of concern blocking the Welfare Reform Bill’s passage through the Stormont Assembly, sparking fears for the future of the political institutions.

“At Stormont House the five parties agreed to a series of measures to protect the vulnerable safeguard current and future welfare claimants under the control of the Executive,” deputy First Minister Martin McGuinness 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. This is totally unacceptable. If the DUP want to strip benefits from children with disabilities, from adults with severe disabilities and 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.”

The Sinn Féin move was the trigger for an angry round of claims and counter-claims, with all the unionist parties, as well as the SDLP and Alliance, accusing Sinn Féin of performing a U-turn as a result of negative feedback from the party faithful at the Sinn Féin Ard Fheis in Derry at the weekend. There have also been allegations that Sinn Féin are willing to sacrifice the hard-won Stormont House Agreement to bring their northern economic policy into line with their policy in the south, which is strongly anti-austerity.

First Minister Peter Robinson denied his Executive colleague’s accusation of bad faith and said the DUP was implementing what was agreed at Stormont House to the letter. And he warned of a coming crisis if the Stormont House Agreement collapses over the new welfare reform impasse.

“We had an agreement,” he said, “we put the figures down so that there was no doubt, nobody can be in any doubt what we agreed to. The Stormont House Agreement would fall because this is a key element of it. If the Stormont House Agreement falls then we are back to a crisis situation. They [Sinn Féin] have decided to bring the show to an end.” He described the Sinn Féin decision to block welfare reform as “dishonourable and ham-fisted”.

A Downing Street spokesman said Prime Minister David Cameron viewed the latest political roadblock as “deeply concerning”. He said Mr Cameron was anxious for a deal on welfare reform to be put in place, but added: “It must be for the locally elected political leadership to find a way forward, the responsibility is with them.”

SDLP leader Alasdair McDonnell said he’s glad Sinn Féin have “woken up to reality” and joined him and his colleagues in their “principled opposition to this attack on the vulnerable.”

UUP leader Mike Nesbitt said the Sinn Féin move had been made with an eye fixed firmly on politics in the south. “For selfish reasons they have put at risk all of the policial institutions,” he said.

But the most stinging attack on Sinn Féin came, perhaps surprisingly, from Alliance leader and Justice Minister David Ford. “Sinn Féin is being either utterly cynical or utterly cowardly or even utterly stupid,” he said. “Everybody was fully aware that the welfare arrangements were only temporary transitional measures.”

Though the political temperature is soaring, there will be no movement in the absence of the two main party leaders. The real work on getting the stalled process back on track begins when the First and deputy First Ministers are back at their desks, most likely on Thursday.