Siptu has called on Dublin Bus not to persist with the introduction of new routes from tomorrow until a dispute which threatens disruption to services is resolved.
The union's branch organiser Willie Noone also contested the company's claim that up to 36 routes could be affected by industrial action taken by his members and drivers who are members of the National Bus and Rail Union (NBRU).
Both unions - representing about 500 drivers - voted decisively in favour of industrial action on Friday.
The drivers say they will not operate new routes because they will be obliged to start, finish and take breaks in the city centre, 11km from the Harristown depot where they are based.
Dublin Bus said there had been lengthy negotiations over the new routes and it had the backing of the Labour Relations Commission.
But Mr Noone insisted the new routes were being introduced unilaterally and his members did not want to take action tomorrow.
"If disruption does occur it will because the company tries to change rosters and working conditions for drivers unilaterally.
"Our members will be reporting to work normally and will do so, unless the company forces them to operate on the new routes.
I believe there is room for further negotiation and the company should avail of the opportunity to explore this option," Mr Noone said.
NBRU general secretary Michael Faherty said that if members were suspended for refusing to operate the new routes, full industrial action would follow.
Dublin Bus posted a message on its web site saying that due to industrial action, services Harristown Bus Garage will be disrupted tomorrow.
"The main areas which will be affected are: Finglas, Swords, Ballymun, Blanchardstown, Donabate, Portrane, Dunboyne, Littlepace, Tyrrelstown, Damastown, Portmarnock, Kinsealy and Kilmore.
The following routes will have no service: 4, 13, 13a, 17a, 27b, 33b, 37x, 39x, 39b, 40, 40a, 40b, 40c, 40d, 70x, 83, 102, 105, 127, 129, 142, 230, 237, 238, 239.
The following routes will have a limited service: 27x, 37, 38/a, 38c, 39, 41, 41c, 41x, 43, 70/a, 270," the posting says.
All other services will operate as normal. An information line has been set up on 01 8734222.