Millions of drivers who may have been have been illegally charged for parking overstays on private land could be eligible for a refund, according to the RAC Foundation.
Private land owners have charged some drivers as much as £100 - and in some cases significantly more - for exceeding time limits. But as the genuine loss suffered by landowners is likely to be significantly less than charges levelled at drivers, the courts could see such payments as penalties – rending them unenforceable.
Professor Stephen Glaister, the RAC Foundation director, said: "Millions of drivers could be in line for a refund. We estimate that in 2013 alone drivers might have been overcharged by some £100 million."
In one incident, a mother from High Wycombe in Buckinghamshire was forced to pay £100 after she ran over time, because she was attending to her upset three-year-old son.
Another case saw a woman from Stokenchurch in Buckinghamshire handed a £100 penalty after she parked outside her partner's home, where she had parked without a problem every weekend for two years.
Drivers are being asked to pay hefty fines, even though clamping on private land was banned by the Protection of Freedoms Act.
In a report for the RAC, barrister John de Waal QC argued that the fines are likely to be several times more than compensation for a genuine loss, and would therefore not be enforceable by the courts.
Mr de Waal said: "Payments at the level that operators presently demand as sanctions are unlikely to count as genuine pre-estimate of loss; they should be seen by the courts as penalties, which means they are unenforceable.”
Mr de Waal added 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".
He also said that when signs are not clear or prominently displayed, the charge can also be challenged on the grounds of unfairness.
Prof Glaister said: "We would like to see this legal argument tested in a higher court so that a binding precedent is set.
"At the same time we would like the Government to do what it should have done at the outset and set out what are reasonable charges."
He went on: "Ministers thought that the ban on clamping would end parking problems on private land. As we warned at the time, they were wrong.
"They allowed a system of ticketing to emerge which is barely regulated. In effect drivers have been short-changed."
A Department for Transport spokesman said: "This Government is on the side of the motorist and we have been clear that we want to see an end to unfair parking practices - including taking strong action to ban clamping on private land.
"Parking in private car parks means that motorists enter a contract with the landowner and the courts must decide if the level of a parking fine is justified should there be a dispute."