Low-paid nurses are among medical staff being made to pay up to £90 a month to use hospital car parks.
Porters and front-line health care workers are having to pay the "extortionate" fees to park in front of their workplaces, a new study reveals.
Many NHS Trusts operate a flat-rate payment system regardless of how much staff are paid, research by Unison shows.
Royal Free London NHS Foundation Trust is one of the top chargers, with full-time staff forking out £85.38 a month to park at the hospital site.
Others, including the Royal Liverpool and Broadgreen University Hospitals Trust, charged £79.50 a month, a Freedom of Information request revealed.
According to responses from some 199 trusts across Britain, a number showed that they offered discounted parking for staff on lower wages, or did not charge healthcare workers to park.
Unison's head of health Christina McAnea described the parking fees for healthcare workers struggling to get by as "extortionate".
"Many NHS staff work shifts so they have to drive because they can't get buses or trains in the middle of the night," she said.
"Health workers in rural areas, where public transport is virtually non-existent, are entirely dependent on their cars to get to work.
"Others have to fork out for expensive permits with no guarantee of a space when they get to work.
"The Government should be guaranteeing fair parking charges for all health employees, and the NHS should stop making money off the back of its dedicated workforce."