The SDLP could pull out of all-party talks and may even refuse to endorse a political settlement, the party's deputy leader has warned.
And there are fears in government circles that the Ulster Unionists are also considering withdrawing as the talks enter a critical phase.
As the British and Irish prime ministers prepare to give the parties a final push at Stormont, the SDLP's Dolores Kelly said that for her party to back an agreement, it would have to be "comprehensive and lasting and deal with all the issues".
"If we don't actually get to that point, then there will be a political call for us in the SDLP to make," she said.
"We haven't decided that yet but the party is clear that we can't be taken for granted. The SDLP has never walked out of any of the talks processes in the past - it is not generally in our mindset - but we won't buy a deal unless it is comprehensive. At the same time we will try to bag anything that is good."
Ms Kelly was speaking in the run-up to what is billed as the final round of talks on flags, parading, the past, welfare reform and the budget. Prime Minister David Cameron and Taoiseach Enda Kenny are expected in Stormont tomorrow, and could stay as long as Saturday if it seems a deal is likely.
Next week Gary Hart, the US Special Envoy to the talks, and Greg Burton, the US Consul General, hope to be in Belfast. Mr Hart has told the Belfast Telegraph he intended bringing a US Congressional delegation and perhaps other American visitors here in the new year if a deal is concluded.
It is generally assumed there will be no further negotiations after new year as the Westminster election campaign will dominate events from that point.
Doubts about the positions of the SDLP and UUP are currently forcing Sinn Fein and the DUP closer together. It is also strengthening the bigger parties' hands in seeking concessions from the two governments.
On Friday morning Mr Kenny met Peter Robinson and Martin McGuinness and said that the Irish government would review its 2011 decision to withdraw £400m funding for the planned A5. The road upgrade from Londonderry to Aughnacloy would link Donegal to Monaghan via Northern Ireland.
Stormont would also be expected to contribute a similar sum and European aid may be available because it is a cross-border project.
The DUP and Sinn Fein are hoping to use the threat of Stormont collapsing to secure Northern Ireland a financial package in addition to powers to set corporation tax rates.
Last night Secretary of State Theresa Villiers said the negotiations were "entering a crucial few days".
"This is the week for Northern Ireland's politicians to demonstrate strong leadership and engage earnestly and positively in the interests of securing stable and effective government for all in Northern Ireland," she said.
Prime Minister David Cameron and Taoiseach Enda Kenny will lead the negotiations from tomorrow in a bid to get a deal over the line. As well as legacy issues, the parties are trying to find resolutions to a range of other disputes including setting a budget and welfare reform.