Four unions opposed to the Croke Park Agreement II proposals have confirmed that they will not be bound by a majority ICTU vote in favour of the deal.
The move is a significant break from traditional practice where minority unions row in behind the majority in such ballots.
The four unions in question - the CPSU, the IMO, the INMO and Unite - are to write to the Public Services Committee of ICTU outlining their decision.
They are also seeking an urgent meeting of the ICTU executive council to discuss the current situation, which was described as "uncharted waters".
The four unions walked out of the Croke Park talks before their conclusion in February.
The decision was made at a meeting this morning.
It was also attended by the TUI, the ASTI and the Association of Higher Civil and Public Servants, which is also advocating a No vote.
The Irish Federation of University Teachers was unable to attend.
CPSU General Secretary Eoin Ronayne said his members would not be comfortable voting for a pay cut for other public servants.
Mr Ronayne said the decision will have to be approved at its annual conference, which takes place in two weeks' time.
Responding to the news Minister for Public Expenditure and Reform Brendan Howlin said it was dishonest to say that there would be no consequences to a No vote in the ballot.
Mr Howlin said the "no" unions were offering only uncertainty to public servants and that the consequences of rejecting the deal would have wider implications for the Irish economy.
INMO General Secretary Liam Doran said this morning's comments by IMF official Ashoka Mody that austerity was the wrong policy vindicated their position.
A spokesperson for ICTU said it was aware of the meeting, but that it had not yet received any correspondence.
He said it could not comment until it saw the arguments outlined in any such correspondence.
Elsewhere, the Medical Laboratory Scientists Association has voted to reject the Croke Park II proposals.
In a statement posted on its website, the MLSA said 59% had voted against the deal, with 41% in favour. The turnout was 76%.
The only other union to complete a ballot so far is the Teachers' Union of Ireland, which voted by 86% to 14% to reject the deal.
The Government has reiterated on a number of occasions that if the Croke Park proposals are rejected, it will legislate to impose cost savings and across-the-board pay cuts of up to 7%.
The result of individual union ballots and the final outcome of the vote will be known by next Wednesday.