More than a quarter of hospital trusts have increased car parking charges for patients and visitors, according to new data.
Hospitals in England make millions of pounds every year from parking, which is free for most people in Northern Ireland, Wales and Scotland.
Data from 197 hospital and mental health trusts found that while some have decreased prices, 28% have upped charges, some by over 100%.
A patients' group hit out at the "alarming" rise in fees, branding them a "tax on the sick".
Stockport NHS Foundation Trust and the Royal Marsden cancer hospital in London charged patients 100% more in 2010/11 than 2009/10.
They increased prices from 50p an hour on average to £1 an hour.
United Lincolnshire Hospitals NHS Trust upped prices 112%, from 67p an hour to £1.42.
North West London Hospitals NHS Trust charged £1.58 an hour on average, up 81% on the 88p it charged the year before.
Northumberland, Tyne and Wear NHS Foundation Trust charged patients 200% more in 2010/11 than 2009/10, increasing prices from 25p an hour on average to 75p an hour.
The figures, analysed by data company SSentif, are provided to the NHS Information Centre by NHS trusts.
Only 16% of trusts reduced their average hourly parking charge for patients and visitors, with 54% making no concession and 28% increasing their charges.
Overall, car parks in hospital trusts in the South West were the most affordable, at an average of 52p per hour.
London and the South East had the most expensive charges at an average of £1.02 per hour.
Separate analysis found some trusts are charging much more than the national average (77p) for an hour's hospital parking, based on the average from a three-hour stay.
Chelsea and Westminster Hospital NHS Foundation Trust charges £2.50 an hour on average, with a minimum of £3.50 for up to one hour.
Ashford and St Peter's Hospitals NHS Foundation Trust in Surrey charges £2 for the first hour, £3 for one to hours and £4 for two to three hours.
Royal Surrey County Hospital NHS Foundation Trust charges £2 for up to two hours and £4 for two to four hours.
Dorset County Hospital NHS Foundation Trust charges £1.60 for one hour, £2.10 for two and £4.20 for up to four.
In September 2010, the Government announced it would not back a pledge made by Labour to scrap car parking charges in England's hospitals, saying the plan could not be justified.
NHS trusts make more than £100 million a year from charging patients and visitors, although the Department of Health has stopped collecting the figures centrally.
Earlier this week, New Cross Hospital in Wolverhampton claimed it needed to put up car parking charges to protect jobs.
Judy Aldred, managing director of SSentif, said: "Despite widespread criticism, the Government took the decision to keep hospital car parking charges in place and to allow trusts to set their own hourly rates.
"Whilst it is encouraging to see that some trusts have removed their charges or reduced them, our comparative study shows that charges still differ wildly across the country and those with long-term illnesses - or those visiting people with a long-term illness - are paying the price."
The Government discontinued collecting data on income from car parking in 2008/09.
Health Minister Simon Burns said: "We expect patients whose healthcare needs require frequent or extended hospital visits to be treated fairly, and car parking policies should not discriminate against them.
"NHS organisations must make the best use of public funds, and this includes setting car parking charges at a rate which is reasonable for the communities they serve and provides appropriate concessions.
"These figures illustrate the fact that three-quarters of hospitals have reduced their charges or left them unchanged."
He added: "No one should be paying extortionate amounts to park in an NHS car park, but introducing free hospital car parking could cost the NHS more than £100 million - money that would otherwise be spent on patient care."
Katherine Murphy, chief executive of the Patients Association, said: "The extraordinary rise in the price of car parking at our hospitals is alarming.
"While Wales and Scotland have scrapped car parking charges altogether, some hospitals have continued to raise their fees.
"Patients and visitors who attend hospital on a regular basis and for long periods of time are being forced to pay these exorbitant charges.
"It is often money patients do not have in these troubled financial times and is a tax on the sick when people have already paid for their health service.
"Hospitals should be properly supported by the Government and should not have to rely on charging patients and visitors to park to make ends meet.
"Car parking charges make a mockery of a service supposed to be free at the point of need."
Duleep Allirajah, head of policy at Macmillan Cancer Support, said: "Cancer is an expensive condition and patients shouldn't be penalised for accessing the treatment they need.
"Public transport is generally not an option as they are undergoing gruelling treatment."