Strikes that would affect up to 334,000 Dublin Bus passengers a day may take place as early as next month as workers pursue at least the same pay rise as Luas drivers.
Fears are growing that work stoppages could take place as schools re-open.
Dublin commuters were hit by extensive Luas strikes earlier this year.
The National Bus and Rail Workers Union (NBRU) has written to its four sister unions at Dublin Bus to meet to co-ordinate a campaign of industrial action when ballots finish next week.
NBRU's national executive met yesterday to discuss a number of options on industrial action including a no-fares day and strikes to win a wage increase of at least 3.8pc a year. The meeting was held after union members backed industrial action by 96pc.
As well as a no-fares day, an all-out strike and 24- or 48-hour work stoppages were discussed.
The meeting came after union members rejected a Labour Court recommendation that they get a wage hike worth 2.75pc a year each year for three years.
They want the same increase as the tram drivers as the first part of a mission to achieve a 31pc increase that would give them equal pay.
Bus drivers' wages are around €39,000 a year including shift and premium payments, but tram drivers stand to make up to €53,000 after the pay increase they won two months ago.
NBRU is the second-biggest union at the bus company, with 1,432 members, who are mainly drivers.
Siptu, Dublin Bus' biggest union, is still balloting its members in seven grades on industrial action, including striking. It has urged them to reject the Labour Court proposal.
Siptu, which has 1,686 members at Dublin Bus including drivers, supervisors, craft and clerical workers, will reveal its ballot result next Tuesday.
The unions submitted a claim for a 5pc pay increase per year for three years, or a total of 15pc, at the Labour Court.
They also sought a retrospective payment to mark a 6pc national pay deal increase that was not paid.
The unions argue that staff are angry and want an improvement in their pay packets after eight years of austerity, which included two cost-cutting plans, pay cuts and voluntary redundancies.
They point out that bus drivers had seen no recession for Luas drivers, as the latter did not have to forfeit payments due under the national pay deal, kept receiving increments and got a moderate pay rise before the one they achieved in June following 12 days of strikes.
NBRU general secretary Dermot O'Leary said the campaign of industrial action could be avoided if the company indicated it was willing to come back to the talks table.
Meanwhile, one of the smaller unions at Dublin Bus representing clerical and supervisory staff, the Transport Salaried Staffs Association, will reveal the result of its ballot on industrial action tomorrow.
Union leader Manuel Cortes said members were unhappy with details of the court's off-er, especially as it meant pay increases, if accepted, would not be pensionable until next January.
"We will have to see what all three unions can now do together to secure improvements from management," he said.
"No one wants to see disruption to the travelling public, and hopefully we can avoid that if management sit down with us to reach a fair and honourable deal. But we cannot rule out industrial action at this stage."
In his recommendation, Labour Court deputy chairman Brendan Hayes said Dublin Bus was recovering from a deep recession and passenger numbers and fares have risen.
However, he said this recovery is in its infancy and must be allowed to develop before the company could support significant wage increases.
Dublin Bus said it would wait for the ballot results and consider the outcome.