Western Sussex Hospitals NHS Foundation Trust, which runs Worthing, Southlands and St Richard’s hospitals, collected £1.23million in the 2014/15 financial year.
Labour party members described the charges as a ‘tax on the sick’, calling for hospital car parking to be made free, in line with Scotland and Wales.
The trust, meanwhile, argued the charges were fair, with certain groups afforded significant discount.
Labour veteran Jim Deen said: “These parking charges are, in effect, a tax on people who are sick at a time when they are often under greatest financial difficulty.
“We pride ourselves on having an NHS that is free at the point of access, but many patients have to pay extortionate hospital car parking charges to access treatment.”
News agency the Press Association lodged a series of freedom of information requests across the country.
Of the 90 trusts which responded, half were collecting in excess of £1million, with seven making more than £3million.
The charges were widely critisised by patient groups, including the Patients Association.
David Jones, director of facilities and estates at the trust, said the charges had not increased for several years and funds were reinvested back into patient care.
He said: “We try to keep the costs of parking as low as possible, and our charges haven’t gone up for five years.
“We also provide significant concessions for patients who are in hospital for a long period, and for specific groups – such as patients undergoing regular dialysis or cancer treatment – we ensure that their parking is free.
“We set our charges to be fair, and in line with other local authorities in the area, which helps to make sure our car parks are only used by people visiting our hospitals, and not the town centre.
“The money raised from car parking at our three hospitals in West Sussex is reinvested by the trust into direct patient care and improving facilities for patients and visitors.”
Despite the trust’s assurances, Mr Deen argued funds should be found from other sources.
“While we are heartened to learn from the trust that the profits from car parking are being used to fund hospital services, this funding should come from general taxation and not disproportionately from those using the services.”
Hospital car parking for patients and staff is free in Scotland and Wales.
Labour says that the NHS in England should follow their lead.
Mr Deen added: “In parliament, Labour has tried to raise the proposal to phase out car parking charges in hospitals in England, but the proposals have been blocked by Tory MPs.”
Among the trusts gathering the highest amounts from car parking were University Hospital Southampton NHS Foundation Trust, which took £3,876,314, and Oxford University Hospitals NHS Foundation Trust, which raised £3,728,000.
A spokesman for the local trust said its car parks were not run by private parking firms.