The National Bus and Rail Union will begin balloting its members today for industrial action in protest at the Minister for Transport's planned privatisation of public transport.
The NRBU general secretary, Mr Liam Toibin, said Mr Brennan had no mandate to introduce changes not agreed in the National Transport Forum last April.
"We have no option but to ballot given the Minister's decision to go ahead and propose privatisation," Mr Toibin said.
"He has ignored us and thrown out the agreed changes made by the National Transport Forum."
One of the changes agreed last April at the forum was that Dublin Bus would retain its core service, but recent proposals by the Department of Transport indicated that 25 per cent of Dublin Bus would be franchised out by 2004.
"The Minister can't throw away 2½ years' work at the forum and start afresh," Mr Toibin said.
"Let's finish the job we've started in the forum. We didn't embark on industrial action easily; it's a last resort," he added.
If the 4,000 NBRU members vote for industrial action, it is likely to occur before Christmas, according to Mr Toibin.
A spokesman for the Department of Transport told The Irish Times that the Minister was looking forward to discussing his proposals with the NBRU at the forum.