Millions of pounds of parking penalties could have been charged illegally, according to the RAC Foundation.
Penalties for overstays in car parks on private land in England and Wales could in some cases be unenforceable in court, the charity said.
It said the penalties were much more expensive than compensation for a genuine loss incurred by landowners.
But the Independent Parking Committee said penalties are the only protection landowners have against losses.
The foundation also called for the government to ensure that extra parking charges were "reasonable and enforceable" and wants to see its argument tested in court.
Its report was compiled by barrister John de Waal QC, who also said parking companies were levying charges on drivers which were disproportionate to the losses suffered by landowners as a result of motorists' actions.
The Protection of Freedoms Act 2012 banned clamping, towing, blocking-in or immobilising a vehicle without lawful authority on private land, in a bid to end abuses by rogue clamping firms.
Independent Parking Committee
However, the foundation said that private car parks were now using overstay penalty charges as an alternative, with a system of ticketing which was "barely regulated".
Drivers who stay longer than the time they have paid for may receive tickets demanding payments of up to £100, and significantly more in some cases, it said.
It cited the case of a woman from High Wycombe who, in 2014, had been penalty charged £100 for overstaying in a car park which cost 20p per hour.
The report also said European legislation, which requires contracts to be fair, meant so-called "early payment discounts" of penalty charges could also be unlawful because they constitute a "price escalation clause".
It said when parking signs were not clear or prominently displayed, the charge could also be challenged on the grounds of unfairness.
Foundation director Professor Stephen Glaister estimated overcharging in private parking penalties may have reached £100m in 2013, and said millions of drivers could be due a refund.
"We would like to see this legal argument tested in a higher court so that a binding precedent is set," he said.He also said a case coming to the Court of Appeal next week, regarding a motorist who is contesting an £85 penalty charge, could establish a precedent in law if the motorist wins, meaning others could have a case to get money repaid.
Source : Click Me