SIPTU members in Bus Éireann and Dublin Bus will meet tomorrow to consider balloting for industrial action.
It comes after the breakdown of talks on plans to privatise 10% of public bus services.
Discussions had been taking place at the Labour Relations Commission for months between SIPTU, the National Bus and Rail Union, the National Transport Authority and the Department of Transport, Tourism and Sport.
However, today the two unions suspended their involvement in the talks.
They said the department and the NTA had failed to give them assurances on what will happen to staff if routes are transferred to private operators.
The routes are subject to an open tendering process at the moment.
The issue has become increasingly important to unions, as the deadline for the receipt of tenders for routes is 7 April.
Earlier, Minister for Transport Tourism and Sport Paschal Donohoe said it was "disappointing" that bus driver unions had withdrawn from the talks.
However, he defended the Government's policy of putting 10% of bus routes out to tender.
Meanwhile, SIPTU Divisional Organiser Owen Reidy said SIPTU and the NBRU would be talking to their respective committees.
SIPTU had sought clarity on issues including whether staff would be forced to transfer to private operators, and what would happen if those operators breached employment rights rules.
They also wanted labour costs to be removed from the tender criteria to avoid bidders putting downward pressure on pay and conditions.
NBRU General Secretary Dermot O'Leary said his union intends to initiate High Court proceedings against the NTA's implementation of the Government's privatisation policy.
Minister Donohoe said he believed that support for improved bus services should be given in conjunction with an appropriate degree of competitive tension within the market, in the interests of quality and cost effectiveness for commuters.
This was why he was committed to the tendering of 10% of bus routes, he said.
The minister said he understood that the LRC negotiations had been progressing, with a number of employee concerns being advanced.
He highlighted that this year, for the first time since 2008, the level of Public Service Obligation contribution for bus and rail services was being maintained at the same level as 2014.
He also noted that he had secured €110m for public transport as part of the 2014 Supplementary Estimate.
Improved public transport, whether new services or increased frequencies of existing services, will require additional funding, he added.