Dublin Bus drivers, represented by the National Rail and Bus Drivers Union (NBRU), have voted to reject a Labour Court pay award.
Earlier this month the Labour Court recommended increases of 8.25% over three years.
However, 95% of members have voted against the recommendation.
Members of the NBRU will be balloted next week on proposals for industrial action.
Meanwhile, SIPTU members in two employment grades in Dublin Bus also rejected the Labour Court recommendation.
Members in the traffic grade, which includes bus drivers, voted by 96% to 4% to reject the recommendation, while their colleagues in the clerical grade voted voted to reject it by 95% to 5%.
SIPTU said the remaining grades are still being balloted and they are expecting a result on Tuesday.
The union will then decide how to proceed.
The Minister for Transport Shane Ross expressed his disappointment at today's ballot outcome and has urged both sides to re-engage as soon as possible with a view to an early settlement.
SIPTU Organiser John Murphy said: "The agreement of the workers to introduce such cost reduction and productivity measures was crucial to the stabilisation of Dublin Bus's financial situation and the company's return to profitability in 2014.
"Our members believe that the Labour Court recommendation does not take into account the sacrifices made by them during this period. They also expressed concerns over other aspects of the recommendation relating to pensions and future productivity."
NBRU General Secretary Dermot O'Leary said: "Our members, by emphatically rejecting this recommendation, have strongly demonstrated their anger at the level of the award compared to that which was awarded to other workers in the same transport sector.
"Whilst our claim for pay parity with tram drivers remains front and centre as part of our goal towards significantly improving the terms and conditions of bus workers, we had hoped that the Labour Court would have assisted towards realising this objective by at least recommending the same quantum which was awarded to resolve the recent Transdev dispute."