More than four in 10 NHS hospitals have increased their prices for car parking in the last year, an investigation has found.
Some NHS trusts have doubled the cost of a stay for patients and visitors.
Freedom of Information data analysed by the Press Association shows that hospitals in England are making more money than ever from charging visitors, staff and patients.
Some 124 NHS trusts responded to the Press Association request for data on parking charges.
Of these, 43% said they had increased prices in the last year for visitors or staff, or both.
Labour has pledged to abolish the costs while the Patients Association said people should not be "charged for being ill".
In some regions, prices have risen sharply, with trusts doubling the cost for some lengths of stay.
Prime Minister Theresa May at Frimley Park Hospital, part of one of the highest earning trusts, earlier this year.
At Airedale NHS Foundation Trust in West Yorkshire, a stay of four to 24 hours cost £8 in 2017/18, up from £3.50 the year before.
The trust made £1,287,322 from parking in 2017/18.
Frimley Health in Surrey, one of the highest earning trusts in England, made £4,452,481 from charging staff, patients and visitors in 2017/18.
This was up on the £4,126,587 it made the year before.
While NHS trusts in England still force people to pay for parking, the charges have been abolished in Wales and most of Scotland.
Some hospitals have defended their revenues, saying some or all of the money is put back into patient care or is spent on maintaining car parks.
Others claim their sheer size and the fact that they serve busy neighbourhoods means they take more revenue.
'A tax on the sick'
Shadow health secretary Jonathan Ashworth said: "These car parking charges are a tax on the sick. The next Labour government will axe them."
Rachel Power, chief executive of the Patients Association, said parking charges generate revenue at a time when hospital finances are "under immense pressure".
But she added: "Charges for car parking at hospitals are a charge on people who are unwell, levied on them because they are unwell.
"We believe that patients should not be effectively charged for being ill."
A Department of Health spokesman said: "We have made it very clear that patients, their families and our hardworking staff should not be subjected to unfair parking charges.
"NHS trusts are responsible for these charges and ensuring revenue goes back into frontline services, and we want to see trusts coming up with options that put staff, patients and their families first."