The UUP deputy leader Mr John Taylor says he will oppose a return to a power-sharing Executive unless more concessions are delivered on policing.
Mr Taylor's remarks will be a blow to his party leader, Mr David Trimble, who has already effectively put his job on the line by trying to persuade his divided party to go back into government with Sinn Fein.
Mr Trimble has tried to swing a majority of his party behind him by insisting that he has negotiated concessions on the name of a new police force as well the right to fly the Union flag from official buildings.
Mr Taylor's support was widely seen as vital when Mr Trimble narrowly won a leadership battle against the Rev Martin Smyth in March, securing just 56 percent of the vote from the UUP's 850-strong policy-making council.
Mr Taylor makes clear in an interview with the Financial Times this morning that he would vote with those opposed to the Belfast Agreement if there was no more movement from the British government on policing.
"I am not interested in what republicans are offering so much as what our own government is offering between now and next Saturday. If there is no change I will be voting against," he told the Financial Times. "I hope [Mr Tony Blair] recognises that there is a majority community in Northern Ireland and that if he wants to make progress he has to compromise."
The newspaper said that after initially expressing enthusiasm for the IRA's offer, Mr Taylor had adopted a more sceptical position, prompting speculation that he was preparing for a leadership challenge should Saturday's vote go against Mr Trimble.
Before Mr Taylor's intervention, political sources said Mr Trimble faced an uphill battle as one-third of the UUP council backed him, another third opposed the deal and the rest were undecided.
Mr Trimble's efforts to sweeten the deal for UUP waverers has already met with opposition from Sinn Fein.
"The IRA initiative, I think, is at risk by the way in which matters have been handled," a spokesman for Sinn Fein told reporters on Monday.
The UUP's attempts to gain concessions outside the bounds of the Belfast Agreement or an independent police commission report last year were unacceptable, he said.