Illegal parking tickets could have ripped off unsuspecting drivers to the tune of millions of pounds for overstaying while parking on private land, the RAC Foundation has claimed.
Some drivers have been threatened with - and many have paid - in excess of £100 for exceeding time limits.
But, the foundation says, if the courts see such payments as penalties, it would render them unenforceable and millions of drivers could be in line for a refund.
Clamping on private land has been illegal since 2012, but the foundation claimed that drivers who overstay are still likely to be slapped with tickets demanding up to - and often considerably more than - £100.
But barrister John de Waal QC, in a paper for the foundation, said the amount was likely to be several times more than compensation for actual loss, meaning it is not enforceable by the courts.
He also said companies and authorities which use 'early payment discounts' to encourage people to pay up early or face higher charged chould be unlawful under European law, because they constitute a "price escalation clause".
The RAC Foundation is now warning more action is needed to tackle the "barely regulated" system.