Sheffield Teaching Hospitals NHS Foundation Trust was highlighted as one of the country's top earners from parking charges after taking £3, 073, 222 in 2016/17 - sparking a row over whether or not the charges should be abolished.
Sheffield Hallam University chancellor lord professor Robert Winston and Sheffield MPs Louise Haigh and Gill Furniss called for them to be scrapped but hospital bosses said the charges were needed as motorists abused the system when it was free.
Unison, which represents scores of NHS workers in Sheffield including nurses and support workers, called for an overhaul of the system.
Charlie Carruth, Unison's Yorkshire and Humberside regional officer, said: "Hardworking NHS staff often have to drive to work – especially given the out-of-town locations of many hospitals and because they finish work long after the last bus or train has gone. Similarly patients visiting dangerously ill relatives cannot always rely on public transport to get them home.
“Exorbitant parking charges by trusts are unacceptable, but the government must take its fair share of the blame. If the NHS were not being so starved of funds, trusts wouldn't have to scrabble around and raise cash any way they can."
However the union representative accepted there was no easy solution to the problem.
He added: "But finding a solution that works, particularly in built up urban areas where it's not possible to create lots of extra parking, is not going to be easy. However charging staff and patients the earth to park, and fining them excessively is not the way forward."
Kevin O’ Regan, director of hotel services for the trust, said: "When we had free parking, this was often abused by people using the car parks for reasons other than visiting hospital or attending for care. This meant there were not spaces available for genuine patients or visitors and important accesses were often blocked."
He added that any income raised is put back into services and does not go to Indigo - a private company hired last year to provide the trust's parking management function for the next five years.
"All the costs incurred to maintain and manage the car parks come from the car park income and not from patient care budgets but if we did not charge, then we would still have to find the costs of providing and managing the car parks from our NHS budgets which would mean considerably less for patient care."
He also responded to claims that there are not enough parking spaces at the Royal Hallamshire and Weston Park - where it costs £8.40 to park for over four hours - and at the Northern General - which costs £3.70 for more than four hours.
Mr O' Regan said: "In the last 12 months we have added another 150 spaces at the Northern General and Jessop Wing."
The data was revealed as part of a wider Freedom of Information request which showed NHS hospitals nationally made a record £174.5 million.