The Civil Public and Services Union, which represents lower grades in the civil service, has overwhelmingly endorsed its negotiators' decision to walk out of the Croke Park talks last month.
The endorsement came at a CPSU consultative conference in Dublin after negotiators presented a detailed report on the proposals to 120 delegates.
General Secretary Eoin Ronayne said the proposals amounted to constructive redundancies, increased hours and further financial pressure on members who had nothing left to give.
The CPSU said it would continue to campaign for a No vote with other unions opposed to the Croke Park proposals at town hall style meetings over the coming weeks.
The result of the ballot of public service unions is expected by 17 April.
The proposals in the deal are aimed at reducing the public sector pay bill by an additional €1 billion over the next three years.