SIPTU is expected to hold a ballot of drivers at Dublin Bus following their rejection of cost-cutting measures last week.
The ballot is to renew their mandate for industrial action in light of what a source called the "changed circumstances".
The SIPTU drivers are now the only group of workers to reject the proposals, which leaves them isolated.
The ballot is expected to take place next week.
The SIPTU drivers had met earlier today to discuss their rejection of the cost-cutting measures.
Dublin Bus is expected to announce a date for implementation of the cost-cutting programme.
The programme has been accepted by all other grades and by drivers who belong to the National Bus and Rail Union.
Dublin Bus has said that it intends to implement the cuts as soon as possible.
It intends to do this given that the overall majority of workers and grades at the company have now accepted the proposals.
The SIPTU drivers rejected the proposals aimed at securing annual savings totalling €11.7m by a narrow margin of 51% to 49%, which was a difference of approximately 20 votes.
However, the National Bus and Rail Union, which represents around half the company's 2,300 drivers, accepted the proposals by a margin of 60%–40%.
Dublin Bus and the Government have consistently stated that the cuts are essential for the survival of the company.
The Department of Transport has already commissioned accountancy firm Deloitte to assess the financial implications for the company if the proposals were rejected.