Sinn Féin has said the current phase of Northern Ireland inter-party discussions at Stormont Castle has run its course.
The party said it will not nominate its Stormont leader Michelle O’Neill as deputy first minister on Monday.
The decision means no power-sharing executive can be formed in time for Monday afternoon’s formal deadline.
“Today we have come to the end of the road,” Mrs O’Neill said.
“The talks process has run its course and Sinn Féin will not be nominating for the position of speaker or for the executive office tomorrow.”
She said the party remained committed to making the power-sharing institutions work.
The parties have until 16:00 on Monday to resolve issues that divide them – if they fail, another snap Northern Ireland Assembly election could be called, just weeks after a poll held at the start of this month.
Sinn Féin leader Gerry Adams said: “We don’t have the terms now to go forward and nominate for a speaker as Michelle has pointed out, or for a first, or a deputy first, minister.
“That’s today. She also said, and I endorse this absolutely, that we do believe that we will have the conditions in the time ahead, because we want to be in the institutions.”
He added: “Will we be back, will we get the institutions in place? Yes.”
Earlier, Mr Adams criticised the DUP and the British government’s approach to the talks.
He said DUP negotiators were not even at the talks on Sunday.
The DUP said that their refusal to negotiate on Sundays is a long-standing policy. They accused Sinn Féin of not being willing to attend roundtable sessions.
Tom Elliott of the Ulster Unionist Party said the current phase of talks was the worst he had been involved in.
“Unless there is a massive u-turn in terms of attitude from the two largest parties, then Northern Ireland could be in for a period of prolonged drift,” he said.
“I understand the secretary of state took the attitude that the blockages to progress were devolved matters and therefore allowed the DUP and Sinn Féin to take the lead, with the government offering support.
“It looks like HM government will have to take the lead at 4pm tomorrow and that is a further indictment of the lead parties at Stormont.”
SDLP leader Colum Eastwood said “rigid opposition to compromise on key issues, particularly from the DUP, has made a comprehensive resolution more difficult to reach”.
He added: “Following talks over the last number of days it is clear now that an agreement will not be reached in the time left. That is a bitter disappointment.
“The secretary of state must immediately create space for all parties to refresh their outlook on the challenges we face and reach a positive accommodation that allows a restoration of power sharing.”
Speaking on the BBC’s Sunday Politics, Alliance Party leader Naomi Long said if there is an agreement on Monday, much of it will have Alliance’s fingerprints on it and that the party will continue to contribute to the talks over the next 24 hours.
“Whether we end up being in government or in opposition, we still want to facilitate government, we want it to happen,” she said.
On Saturday Mrs Long said another election would be a vanity project.