Bus Eireann has said its financial crisis will deepen if drivers follow through with industrial action over pay cuts.
As members of the National Bus and Rail Union (NBRU) warned of stoppages, company management said failure to agree reductions in overtime and expenses will lead to losses of 16 million euros a year.
A Bus Eireann spokesman said: "Any industrial action would be counter-productive for customers, the company and staff. All it would achieve would be to worsen our financial situation, which would then require us to seek even further savings in the area of terms and conditions."
The NBRU said it had put Bus Eireann on notice of industrial action as the company is to press ahead with changes to pay and working conditions from next Sunday.
General secretary Michael Faherty said Bus Eireann had walked away from talks and was unilaterally imposing a pay cut. He said: "To impose conditions without first exhausting the established normal industrial relations process is regrettable and the company have questions to answer in this regard."
The two sides failed to reach agreement at the Labour Relations Commission on a cost-cutting plan but the dispute has not been brought before the Labour Court.
Bus Eireann revealed its financial crisis last June with the biggest savings expected to be made in cuts to overtime and shift rates. The company spokesman added: "It is regrettable that these steps have to be taken. However, they are necessary in order to protect basic pay and jobs."
Bus Eireann's cost-cutting plan includes reducing overtime rates from 1.5-1.25 times; the working week for clerical and executive staff moving from 36 to 39 hours; reducing annual leave by three days this year and in 2014 and 2015; and a one-third cut in other allowances and expenses.
The company has around 2,500 staff, with about two-thirds employed as drivers.
Siptu is balloting its members over possible industrial action over the cost-cutting plan with a result due on Thursday. Under industrial relations rules, the union must give at least one week's notice for industrial action while the company said it has an in-house agreement for 30-day notice.