The dispute between Peter Robinson and Martin McGuinness over welfare reform has deepened with any prospect of progress postponed until June, after the council and European elections.
Relations between Stormont's top two soured further after the First Minister accused Mr McGuinness of failing to sell an agreed package to his Sinn Fein party in May of last year.
In his first intervention since the DUP went on the attack over the stalemate this week, the Deputy First Minister accused Mr Robinson of "crossing the line".
Returning from Colombia, where he had been invited to share the lessons from the Northern Ireland peace process, Mr McGuinness counter-charged the First Minister with "grandstanding" and failing to deliver.
"I think he crossed the line and I think it was a big mistake for him to do that," he said.
"Quite clearly some of the things that he said in relation to the discussions that took place between himself and myself on the issue of welfare cuts bear no reality to what happened at the time. I take umbrage at that." The Deputy First Minister said he had raised welfare cuts directly with Prime Minister David Cameron at Downing Street, and twice with Deputy Prime Minister Nick Clegg in the Cabinet Office.
"Peter Robinson never offered one word of support for a rethink on welfare cuts and the only support I received was from my Scottish colleagues," he said.
"When Peter Robinson talks about a failure to deliver, we should remind ourselves that (he) failed to deliver on the Education and Skills Authority and the Maze/Long Kesh Peace Project – both of them Programme for Government commitments which he and his party signed up to.
"Peter Robinson also failed to stand united in Government with me in condemnation of UVF-orchestrated violence, particularly in Belfast."
And Mr McGuinness said his power-sharing partner had failed to deliver over the Haass proposals on flags, parades and the past, while his party remained oblivious to the £30m policing bill for Twaddell Avenue protest in north Belfast.
"Now they want to hand back powers on welfare," he added.
"The only way to resolve these issues is by mature leadership, sitting down with Executive colleagues and finding a resolution rather than grandstanding on the media and presuming to speak for me," he said.
Mr Robinson, who warned responsibility for benefits could be taken back by Westminster, revealed Mr McGuinness asked him for a meeting on May 8 to say Sinn Fein would not consider a package agreed at OFMDFM level and with other departmental ministers.
"I made it very clear that no deal is done until we get the support of our parties, that we were all comfortable enough to go with the package to our parties," he added.
"That's the position that I outlined, it's the position that I stand over.
"Indeed, in terms of them not being able to get their party over the line, why on Earth would he have brought me in on a Saturday afternoon if there hadn't been some exceptional set of circumstances."
Alliance's Stewart Dickson said the two nationalist parties would be forced to accept reform: "When Westminster takes further money from the Northern Ireland block grant, the SDLP and Sinn Fein will be forced to come to the table and introduce welfare reform."
STORY SO FAR
A letter from Treasury secretary Danny Alexander to Stormont Finance Minister Simon Hamilton warned Northern Ireland will lose £105m from its block grant this year over the failure to implement welfare reform.
In a long-running row, the DUP insists the consequences of non-compliance will be a devastating impact on the delivery of Government services, while Sinn Fein and the SDLP demand further negotiations to prevent vulnerable families being hurt.
Martin McGuinness and Peter Robinson are now in open conflict over whether or not they had made a deal on reform.
The DUP has known for some time there is a lack of consensus over welfare reform.
Which is why their Social Development Minister, Nelson McCausland, withdrew the legislation from the Assembly last summer.
Yet in October of the year before, the Assembly actually voted in favour of the national upheaval towards so-called 'universal credit'.
MLAs sat until just after midnight, passed the bill by 60 votes to 42 and a Sinn Fein move to postpone a decision was defeated.
Then the Social Development Committee, chaired by inn Fein's Alex Maskey, trebled its weekly meetings to examine the legislation in detail, clause by clause.
Their lengthy deliberations showed many more amendments which the parties, including unionists, wanted to make to ensure a smoother delivery of the changes. As ever, the devil was in the detail.
The final upshot was that Minister McCausland pulled the legislation before it could reach 'consideration stage', citing a lack of consensus.
Now the DUP insists there is no realistic prospect of new meaningful negotiations with the Government.
Sinn Fein and the SDLP insist tough negotiations with Downing Street could yet yield more concessions on how the reforms are implemented here.
The biggest shake-up in the 'welfare state' in 50 years will impact on thousands of families here.
By Noel McAdam
No doubt at some point next week we'll see a photo opportunity for our First Minister and Deputy First Minister, out together and doing the business.
But behind the scenes their two parties have been spoiling for a fight for some time and tensions are being exacerbated by the looming elections next month.
A sometimes fraught but always workmanlike relationship between Peter Robinson and Martin McGuinness has been increasingly frayed since their agreement over the Maze Conflict and Resolution Centre fell apart last summer.
Continued and trenchant failure to reach a final compromise on the long-delayed Education and Skills Authority also feeds their mutual frustration.
Then bubbling away in the background for months has been the face-off over welfare reform with this week both the First Minister and Deputy First Minister entering the ring to slug it out.
Despite any rumoured deals over reform, Sinn Fein cannot be seen to be implementing austerity in the North while its main focus is on gains in the European elections in the Republic. The DUP has attempted a carefully choreographed pincer movement to keep Sinn Fein on the back foot, knowing that cutting benefits is an issue popular with its own voters.