Sinn Fein is prepared to collapse Stormont in favour of votes in the Republic and its all-Ireland political ambitions, Ulster Unionist leader Mike Nesbitt has claimed.
Mr Nesbitt has challenged Deputy First Minister Martin McGuinness to declare his party's stance on welfare reform here ahead of the negotiations, which will get underway again tomorrow.
As Sinn Fein was urged to declare its "bottom line" as part of the all-party talks on the budget, welfare reform, parading and the past, Secretary of State Theresa Villiers warned that the corporation tax deal on offer would be delayed until after a general election in May if agreement could not be reached this week.
Mr Nesbitt spoke of his fears that a possible collapse could be triggered by Sinn Fein in the absence of movement.
He asked several questions - who do they want to protect from welfare reform, how do they want to do it and what will it cost?
The UUP leader said: "They obviously have an eye on what reaction they are going to get in Galway, Limerick and Dublin.
"If they don't answer my questions, then inevitably you are going to be forced to the conclusion that they are prepared to collapse Stormont."
He continued: "I've no idea how likely this is, but Martin McGuinness can do it in a heartbeat. All he has to do is resign and that triggers an election."
He spoke of his frustration at how Mr McGuinness blanked him last week after the Prime Minister left the talks and accused Sinn Fein's most senior politician at Stormont of looking at the people in the Republic rather than those in Northern Ireland.
He added: "The ultimate nonsense is that Sinn Fein have a political policy of Brits out and an economic policy of Brit money in.
"The Prime Minister had money in his back pocket no doubt; he put a little bit on the table but he had more.
"If they are really concerned at helping the vulnerable, then they have got to declare their bottom line as there is more money to come out of London."
Ms Villiers yesterday spelled out the threat of losing devolved corporation tax powers resulting from a failure to reach an agreement.
Mrs Villiers said: "What happens at the end of a parliament is, essentially, there is a wash-up stage where government and opposition discuss what can be speeded through so that it's done before dissolution takes place. It's not completely impossible that corporation tax could still be achieved in this parliament, with a later introduction after this week, but it becomes increasingly difficult."
Finance Minister Simon Hamilton said that he did not think anyone involved in the talks fully understood Sinn Fein's position on welfare reform.
He said: "Sinn Fein's position is an impediment to agreeing a budget and if there's no budget agreed then governing becomes impossible."
Alban Maginess of the SDLP was still optimistic that agreement could be reached.
"I think the outstanding issue is not welfare reform but rather the budget and the block grant that we get from Britain. I think it's time that we reach agreement because we cannot run the Assembly and the Executive without stability and confidence and the best way to get this is to get agreement."
Alliance leader David Ford said he believed that "lessons could be learnt" from last year's failed talks with US envoy Dr Richard Haass.
"I think it (a deal) can be done if people are prepared to put their minds to it and the energy into it that the process requires," he added.
Sinn Fein's Gerry Kelly said: "The deal that should have been done, should have been done in the last few days.
"Though I wish I could tell you I was hopeful, I can tell you that we will work hard to see if that will occur, but unless the British Prime Minister changes his attitude and that ideological doctrine of cuts no matter what, then I think we are in difficulty."
One analyst said while the talks are technically still alive, they are in urgent need of rescuing and it is a race against the clock, and if progress is not made before Christmas, then it probably won't happen before the May General Election.
Story so far
Prime Minister David Cameron left the Stormont talks last week promising the Assembly £1bn of "financial firepower" if political parties could agree on a range of issues such as financial and welfare reform, flags, identity and culture, parades and the past. A final decision on devolving corporation tax powers will also depend on the outcome of the talks.