A ballot of SIPTU drivers at Dublin Bus has begun on whether to take strike action if the company implements cost-cutting proposals.
Last week, SIPTU drivers narrowly rejected by a margin of 51% to 49% the latest proposed cost-saving measures.
All other Dublin Bus grades, including half the drivers who are represented by the National Bus and Rail Union, have accepted the cuts, which followed an independent report into the company's future.
The board of Dublin Bus has said it will proceed with implementing its cost-reduction plan on 17 November.
The Government has said it will stand by Dublin Bus in the event of industrial action.