A third of hospital trusts in England have increased their car parking charges in the last year, figures show.
Some are now charging as much as £4 for a one-hour stay, with a third increasing their average charge over a three-hour period.
The analysis, by the Press Association, includes figures obtained directly from NHS trusts and data submitted to NHS Digital.
The NHS Digital data suggests a 15% average rise in parking charges across trusts in England between 2014/15 and 2015/16.
Some trusts allow patients and visitors to park for free for the first 30 minutes before charges kick in.
But others have scrapped a one or two-hour charge, meaning people have to pay a flat fee for three hours even if they are only staying for 45 minutes.
The most expensive trust in the country for a one-hour stay is the Royal Surrey County Hospital in Guildford, where patients are forced to pay £4 for any stay up to two hours. It does have a few bays where people can park for 20 minutes before being charged.
Hereford County Hospital in Herefordshire lets people park for free for 10 minutes, but then charges £3.50 for an hour and £5 for two hours.
Meanwhile, the Royal Free Hospital in London charges a flat rate of £3 per hour, 24 hours a day, seven days a week.
Of the 209 hospital trusts that reported figures to NHS Digital for both 2014/15 and 2015/16, a third (69 trusts) showed an increase in their average hourly charge when calculated across three hours.
Some 60% (126) showed no change over the year, while 7% (14) showed a decrease.
Among those trusts that have hiked up parking charges in 2016 are the Royal Surrey, where the cost of an hour has doubled from £2 in 2013 to £4 now.
Stockport NHS Foundation Trust increased its prices by about 40% over the summer.
The cost of a four-hour stay at Stepping Hill Hospital rose from £6 to £8, with a short stay rising from £2.50 to £3.50.
England is the only part of the UK where hospitals routinely charge patients and visitors for parking.
Almost four out of 10 (38%) NHS trusts said they also charged for disabled parking.
Laura Keely, campaigns manager at Macmillan Cancer Support, said: "Cancer patients often need to make frequent trips to hospital. They should not be left out of pocket in order to receive life-saving treatment.
"Public transport is not always an option as cancer patients can have an increased risk of infection because treatment has compromised their immune system.
"Treatments can also leave patients feeling tired, sick and weak, so they have little choice but to travel by car.
"Car parking is free at most hospitals in Scotland, Northern Ireland and Wales but not across all hospitals in England.
"Those in charge of hospital car parking have a responsibility to ensure that their schemes work for vulnerable people, such as those living with cancer."
At the other end of the scale, car parking at Trafford General Hospital in Greater Manchester is free for up to three hours.
Alder Hey Children's NHS Foundation Trust charges £2 per day and the Christie cancer hospital in Manchester charges £1.50 per day. The Clatterbridge Cancer Centre on the Wirral offers free parking.
Some NHS trusts offer concessions to visitors whose loved one is terminally ill, and also discounts or weekly tickets for lengthy courses of treatment.
Last December, figures obtained under the Freedom of Information Act by the Press Association revealed that some NHS trusts are making more than £3 million a year from car parking fees.
Of more than 90 trusts that responded to the FoI request, half are making at least £1 million a year.
Seven NHS trusts earned more than £3 million in 2014/15 from charges, a further eight made more than £2 million a year while a further 33 earned more than £1 million a year.
The most expensive trusts in England for a one-hour visit are:
:: Royal Surrey County Hospital £4
:: Hereford County Hospital: £3.50
:: Stockport £3.50
:: Bristol Royal Infirmary £3.40
:: West Suffolk Hospital £3.30
:: Northampton General £3.10
:: Royal Free, London £3
:: Basildon Hospital, Essex £3
:: Whittington Hospital, London (after 5pm) £3
:: St Thomas' Hospital, London £3
:: Chelsea & Westminster Hospital, London £3
:: Aintree University Hospital £3
:: Luton and Dunstable £3
:: Mid Cheshire Hospitals £3
:: Mid Essex £3
:: Southend University Hospital £3
:: Princess Alexandra Hospital, Essex £3
:: University Hospital of South Manchester £3
:: Warrington Hospital £3
Royal Surrey had a deficit of more than £11m at the end of March, against a forecast of £2m. It has blamed several factors including increasingly expensive agency staff.
It said it was expanding its car park capacity by 100 spaces due to increased demand. A £10m multi-storey is planned to create a long-term solution, it said.
Deputy chief executive Alf Turner said: "I do not like having to charge people for car parking and in an ideal world we would not have to, however both my colleagues on the board and I know that it is an unfortunate necessity to cover our car parking improvement and running costs.
"We have listened carefully to the views of our patients, visitors and staff and they have been telling us that our current car parking provisions are inadequate and urgent changes need to be made.
"The Trust's challenging financial position means that it does not have the additional funds to invest in the future of its car parking facilities without making the incredibly difficult decision to charge people for using the car parks."
Shadow health secretary Jonathan Ashworth said: "These figures show a worrying increase in the cost of car parking charges in our hospitals.
"Racking up charges on people who have no choice isn't fair and will only cause more distress for patients and their families.
"Hospitals across the country are hard-pressed because of this Government's underfunding of the NHS, but money should not be made up through charging patients and their families more and more for an essential service. These increases cannot be justified."
Katherine Murphy, chief executive of the Patients Association, said: "Why is it that England is the only place still charging for parking?
"It's wrong and it's unjust.
"These charges can put people off visiting hospital. People on low incomes cannot afford some of the exorbitant charges at these trusts.
"There is no national policy and all these trusts are doing what they like.
"We acknowledge that many hospitals are in financial difficulty, but making money out of the public in this way is not the answer."