Motorists could be in line for payments of “tens of millions of pounds” for unfair parking fines of more than £100 a time, according to the RAC Foundation.

John de Waal QC, a barrister, has prepared a report for the motoring organisation arguing that private parking companies are levying huge charges on drivers out of all proportion to the losses suffered by landowners.

Although clamping on private land has been banned since 2012, drivers who stay longer than the time they have paid for can receive tickets that demand payments sometimes significantly more than £100.

“Payments at the level that operators presently demand as sanctions are unlikely to count as a genuine pre-estimate of loss. They should be seen by the courts as penalties, which means they are unenforceable,” de Waal said.

He also argued that European consumer legislation which requires contracts to be fair means so-called “early payment discounts”, which are often used to put pressure on the public to pay up quickly, or face a higher charge, are in fact unlawful because they constitute a “price escalation clause”. And when signs are not clear or prominently displayed, the charge can also be challenged on the grounds of unfairness.

A case being heard in the court of appeal next week could pave the way for the repayment of millions of pounds worth of fines and render any future fines unenforceable.

Barry Beavis was sent a charge letter from Parking Eye after he overstayed a two-hour limit at Riverside Retail Park car park in Chelmsford in April 2013. He contested the initial £85 charge and will appeal the charge next week.

“If the courts agree with Mr de Waal then millions of drivers could be in line for a refund. We estimate that in 2013 alone drivers might have been overcharged by some £100m,” said Professor Stephen Glaister, director of the RAC Foundation.

There is no definitive figure how many private car parks there are in England and Wales, as not all are operated by the trade associations, but it is thought the number is in the region of 20,000. There are no comprehensive turnover figures for the private parking sector but the RAC suggests it is in excess of the £1.43bn a year.

The RAC Foundation report criticises ministers of the current and former governments for not doing more to clamp down on the level of fines charged by private operators.

“They [ministers and officials] have ignored the impact on motorists who have received demands for large amounts in extra parking charges from operators,” said Jo Abbott of the RAC Foundation and co-author of the report. “In particular, they have missed the opportunity to enshrine in legislation some means of interpreting the legal basis for limiting the level of extra parking charges by specifying a clear method of calculating them – a method which precludes unfair overestimates of loss incurred.

She added: “Somewhere along the line they failed to foresee the danger of business models arising within the parking industry that might use payments from those who contravene their regulations to do more than merely compensate them for the cost of the transgression.”

• This article was amended on 20 February 2015 to clarify the attribution of quotes