The Unite trade union has decided to recommend that its 6,000 public service members should reject the Haddington Road Agreement on pay and conditions.
It brings to four the number of unions who have confirmed their continuing opposition to the proposals, which are aimed at reducing the State payroll bill.
The move follows a meeting today of Unite's public sector representatives.
The union is to ballot its members on the deal by 12 June.
Unite Regional Secretary Jimmy Kelly said that after the ballot the union will decide how best to proceed in the best interests of members in light of the Government’s legislation to impose pay cuts.
He said Unite members could rest assured that the union would ensure they were not left vulnerable to what he called the "draconian provisions of the Government's anti-union legislation".
He added that workers were not deceived by what he termed a "Haddington Road label slapped onto a Croke Park bottle" - which they had rejected by 84%.
He noted that there had been no movement on what he called the core issue of withdrawing €1bn from a fragile economy by 2015.
During talks at the Labour Relations Commission, he said Unite had pointed out that the €300 million in cuts being sought for this year would have almost no impact on the deficit.
He said Unite believes those cuts would shrink GDP and cost up to 1,500 jobs, half of which would be in the private sector.
He said the Haddington Road proposals still represented a bad deal for public sector workers, for communities, and for the economy - and called for the €300m in cuts for 2013 to be replaced with a €300m special investment programme.
The unions who remain opposed to the Haddington Road deal are Unite and the Civil Public and Services Union (which are balloting their members), as well as the Teachers' Union of Ireland and the Association of Secondary Teachers in Ireland, who say there are insufficient improvements in the Haddington Road deal to warrant a re-ballot.