SIPTU has warned that a dispute over Bus Éireann's decision to implement changes to terms and conditions without agreement could spread to Iarnród Éireann and Dublin Bus.
Bus Eireann has said it was proceeding with its cost-reduction measures from Sunday.
Speaking after 91% of members voted for action, SIPTU’s Willie Noone said: "If the company proceeds on its current course it is likely that workers across the CIE group will be forced to take similar action.
"Bus Eireann workers are not prepared to continue to provide a public transport system while management is eroding every aspect of their conditions of employment, including their wages."
The National Bus and Rail Union announced it was backing industrial action two days ago.
Bus Éireann said it has projected losses of €16m for 2013 if it does not take action to address costs.
The company is seeking savings of €20m. Up to €11m in savings is to be sought in operational measures and €9m in savings from payroll.
Negotiations at the Labour Relations Commission have failed to break the deadlock.