Car parks across the UK have been labelled cash cows following a study which revealed the lowest parking charges are available in Berkshire.
Parking for 10p per hour is available in Reading's Chester Street car park, compared to an average of £2.29.
The most expensive shown by the survey, of 300 car parks by the GMB union, is £7.30 per hour in central London.
The union said its findings proved private firms are milking money from car parks.
Both the cheapest and most expensive car parks are owned by a private company, National Car Parks (NCP), who declined to comment on the charges.
The most expensive car park in the survey, compiled with GMB union members, was in London's Berners Street.
GMB, a union for car park workers, said its survey showed car parks were being used to extort money.
Rob Kelsall, a GMB official, said: "The charges demonstrate the extent to which car parks are being used to extract private money from motorists.
"Private equity companies have targeted car parks as a cash cow and have taken over £1bn out of the industry in the last few years.
"While this money has been taken out, the numbers employed in each car park has been reduced as workers have been replaced by pay machines, automated barriers and CCTV cameras."