The Prison Officers' Association has overwhelmingly rejected proposals from the Department of Justice aimed at reducing the €64m a year overtime bill.
The result was 2,914 votes against the scheme, and 28 for.
The General Secretary of the POA, John Clinton, said any resolution of the row would have to meet the needs of all parties.
He said prison officers were prepared to enter into further talks under a third party. Earlier, the POA said the deal would involve difficulties for staff on issues including pensionability and excessive flexibility in rostering.
Complex negotiations
Prison officers have been in complex negotiations with the department for some time over a new system of annualised hours.
Under the proposed system, the 3,300 prison officers would receive an annual payment of €10,300 if they agreed to work 360 additional hours a year.
Officers would also receive a €12,250 lump sum compensation payment for the changeover payable over a three-year period.
However, the lump sum would be conditional on verification that agreed cost reductions had taken place.
Justice Minister Michael McDowell has already said that the current cost structure of the Prison Service is unsustainable, and that if the crisis is not resolved by consensus, he will deal with it unilaterally.
