Council staff in Swansea face paying for car parking at work.
The authority says it could raise £250,000 a year by charging employees to park at its main offices and public car parks.
The cabinet is being asked to approve a sliding scale with permits ranging from £99 to £399 depending on salary.
But Unison claims the scheme is unwelcome at a time of pay freezes and discriminates against those working in the main council offices.
Some other councils in Wales, including neighbouring Neath Port Talbot, are introducing similar schemes.
If approved staff in Swansea will be charged for permits for car parks at the Civic Centre, Guildhall and Penllergaer and at council run public car parks they currently use for free.
There would a £99 annual charge those earning less than £15,000 with a sliding scale going up to £399 for those earning more than £50,000.
Cabinet member for the environment John Hague said the proposals had been modified to take account of some union concerns.
"The council faces budget pressures in the current year and in future years," he said.
"The money raised is to assist with the budget and to reduce the need for service or job cuts."
But Unison said the plan was discriminatory as staff working at the main buildings would have to pay where as those at satellite offices would not.
It said staff at Penllergaer would be hit as public transport links to the site were poor.
It also said it was unfair that while councillors themselves would have to pay for permits they would be able to claim back the cost on their expenses.