Jim Nicholson, chairman of James Molyneaux's Ulster Unionist Party, said: "The lines are clearly drawn and I cannot see why it is in the interests of the Unionist people to maintain in power a government so weak that it has lost its authority."
His warning will be doubly worrying for ministers, who regard him as a moderate. Mr Nicholson added: "From here on in, Unionism is on its own. We have no true friends in either government. We expect no quarter but we will not acquiesce in our own destruction."
Mr Molyneaux, leader of the UUP, also spoke out, telling John Major in the Commons not to call the Unionists' bluff about seeking to bring down the Government.
However, it is likely to mean that Mr Major will face debilitating months of defeats in the Commons without being forced into a general election. The nine whipless Tory MPs would be expected to support the Government in a confidence vote, and Labour have said they would put the peace process before party politics.
The warning came as Ulster Unionists stepped up their campaign against the Anglo-Irish framework document proposing a cross-border body. They released the text of a letter to Mr Major after a meeting with the Prime Minister making it clear the cross-border body was unacceptable and the surrender of Irish constitutional claim to the North would not make it palatable. "The creation of all-Ireland political institutions with governmental powers to treat Ireland as one unit for any matter is the antithesis of Unionism.
"Co-operation and even cross-border bodies, properly defined, are one thing. All-Ireland bodies are quite another matter. And no amount of tinkering with the Irish constitution can make such a contradiction of Unionism palatable," the letter said. Ministers, who are pressing ahead with the document, saw one crumb of comfort in the letter, which stopped short of outright opposition to the proposals. It said the Unionists would "wait and see". But Mr Nicholson's remarks raised the prospect that the Unionists will refuse to enter the cross-party talks on the document.
Sinn Fein's president, Gerry Adams, accused Unionist MPs of trying to panic Protestants. The Unionist people should resist those efforts, he said, adding: "All thinking people know that there has to be a change in the status quo if a lasting settlement is to be agreed."
Sir Patrick Mayhew, Secretary of State for Northern Ireland, briefed the Cabinet on the negotiations with Dick Spring, the Irish Foreign Minister. They will have a further meeting, probably on Saturday in Dublin. Downing Street officials said a special Cabinet meeting could be held on Tuesday to approve publication of the document on Wednesday, if they can overcome the final differences.
Ministers are seeking to end speculation by accelerating publication of the document. They were reminded of the threat that the IRA could restart the violence, if progress was not made. Sinn Fein's northern chairman, Mitchel McLaughlin, said: "I actually don't believe that we have created the conditions which will actually ensure that there will be no return to political violence."