Hospital staff are to be balloted on strike action in an ongoing row over car park charges.
Nurses and hospital staff claim they are being taxed by parking
Unison plans to ballot staff employed by NHS Greater Glasgow and Clyde who are affected by parking charges.
The health board began introducing the hospital parking charges for patients, staff and visitors in July.
A recent review saw cuts in charges at many sites but Unison said staff remained angry about the policy, which it described as a tax.
Regional organiser Matt McLaughlin said: "We told NHS Glasgow and Clyde that this decision would be as popular as the Poll Tax and we've been proven right again.
"Our members are very angry over these charges and they are even angrier that their employer has refused to listen and has pressed on with these proposals to tax staff for coming to their work.
"It is unacceptable that vital health care staff have to pay up to £140 per month for parking and I believe that they are prepared to take action to make their employer listen."
New arrangements were recently introduced at Glasgow hospitals, cutting the maximum daily parking charge from £12 to £7.
The number of patients and visitors qualifying for free parking was widened and a new sliding scale for staff permits was created, reducing the monthly charge to £5 for the lowest earners.
Mr McLaughlin said he remained hopeful that a dispute could be avoided and urged NHS bosses to meet with union officials.
He said: "I've written to the employers today, asking them to suspend this scheme and to meet with Unison in an attempt to reach a conclusion that helps them manage problem parking on specific sites, but does not tax staff, patients and visitors in the process."
A statement released by NHS Greater Glasgow and Clyde said it was "disappointing" that Unison would threaten patient care by balloting for strike action.
It added: "It is untrue and unfair to say that staff concerns were not listened to in the process of developing our car parking policy.
"Unison and others made a submission in our 2005 consultation and their concerns, and those of others, helped trigger a review of proposed car parking charges earlier this year."
The health board said its primary objective was to ensure that patients, visitors and staff who need a car to carry out vital NHS duties could park at our hospitals when they needed to.
It said: "As an employer we have no obligation to offer all staff free parking without any constriction."