A new study reveals low-paid nurses and porters are being charged "extortionate" fees of almost £90 a month to use hospital car parks.
Research by Unison showed that many NHS Trusts operated a flat-rate payment system regardless of how much staff were paid.
A Freedom of Information request by the union found the Royal Liverpool and Broadgreen University Hospitals Trust, charged £79.50 a month.
The Royal Free London NHS Foundation Trust is one of the top chargers, with full-time staff paying £85.38 a month to park at the Royal Free Hospital site.
Responses from 199 trusts across Britain also showed that some trusts offered discounted parking for staff on lower wages, or did not charge healthcare workers to park.
Unison's head of health Christina McAnea said:
"Health staff are struggling to get by on a pay rise well below the cost of living and these extortionate fees are an extra tax on their wages.
"Many NHS staff work shifts so they have to drive because they can't get buses or trains in the middle of the night. 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."