SIPTU staff at St James's Hospital in Dublin have voted overwhelmingly to take strike action over management proposals to charge them for parking at work.
Sources said 92% of those who voted backed the industrial action up to and including strike action.
SIPTU has between 850 and 900 staff at the hospital.
In a statement, the union claim parking would cost staff €500 a year.
The union's Health Divisional Organiser said that the attempt to impose such "an excessive car parking charge" is "totally unacceptable".
Paul Bell said that "most of these workers have no option but to drive" to work.
He said a decision on what form the industrial action will take is to be made in the coming days.
He added it would then be up to hospital management to decide whether to proceed with the plan. If they did, the union would respond in kind.
The result of a similar ballot by Impact will also be known in the near future.
Hospital management have said they are aware staff have been balloted, but they are awaiting formal notification of the results.
They say they will continue to engage with staff in the interim.