Drivers are being hit with unfair penalty charges of up to £100 for 'trivial' offences at car parks run by private firms.
Trading Standards officers carried out a probe into the operators following a wave of complaints against unscrupulous practices.
Following a series of spot checks across the country they found drivers have been hit with unenforceable penalty charges ranging from £15 to £100, while unclear signs leave drivers in the dark about what constitutes an offence.
Private car parking at retail centres, supermarkets and standalone private car parks are subject to general consumer laws.
When drivers pay for private parking, they enter into a contract with the provider and are protected by a range of laws aimed at ensuring fairness for consumers, no different to buying goods and services from shops on the ‘high street’.
One-quarter of car parks that officers visited had inadequate signs and malfunctioning ticket machines.
Evidence of unfair penalties was also uncovered, including the case of one driver who paid the correct fee for parking, but was still fined £100 after they failed to type their vehicle registration correctly into the ticket machine.
Around half of car parks use automatic number-plate recognition technology to track vehicles, while some did not inform drivers that payment was required at all. The average fine levied on drivers was £75.
Peter Adamson, chairman of the Society of Chief Officers of Trading Standards in Scotland (SCOTSS), said: "The main problems we found were inadequate signage and ticket machines that were difficult to use or malfunctioning.
"We also found evidence of unfair application of penalty charges, for example a £100 charge for a very minor mistake by a driver, who paid the correct fee for parking but failed to type their vehicle registration correctly into the ticket machine.”
He added: "Private car parking is a legitimate business activity. High demand parking space in our towns and cities must be managed and consumers cannot expect to park for free in these busy urban areas.
"However, at the same time consumers must be treated fairly. They must be given all the information they need to use a car park and must not be subject to unfair terms or practices.
"We want to see an improvement in standards and are working with the industry to achieve this across Scotland. We are working with Government, trade bodies and consumer bodies to improve the lot of Scotland’s drivers when utilising private parking and we would like to see a single Code of Practice developed which addresses these issues.
The Trading Standards probe has been welcomed by the Institute of Advanced Motorists, who say drivers often face "unfair" charges.
Neil Greig, IAM Scottish spokesman, said: "It's great news that long-suffering drivers in Scotland have a supportive ally in the form of their local trading standards department.
"Many drivers have struggled to appeal on their own against unfair charges or unclear signposting so involving the consumer law experts at your council will be a great help to those seeking redress."
Last month Tory MSP Murdo Fraser published plans for legislation to curb the companies' ability to rip off drivers.
His proposals include a cap on the fees that can be slapped on motorists, likely to be set at £60, and an independent body to adjudicate on appeals.
The legislation, which will go to public consultation before being introduced at Holyrood, also proposes new rules on signage in car parks and the removal of 'misleading' terms on fines.
Despite the problems highlighted by SCOTSS, recent court cases have seen judges come down on the side of parking companies when fines have been contested.
Last year Carly Mackie, 28, was given Britain's largest ever parking penalty of £24,500 by a sheriff who warned against the belief that private parking charges are unenforceable.
She parked outside her mother's home in Dundee and ignored the almost daily parking charge notices on her windscreen.
Miss Mackie wrongly believed she could park her Mini on the spot in front of her family's garage near their Dundee home and thought the tickets were unenforceable.
But Vehicle Control Services, a private firm, took her to court when she racked up a huge bill for ignoring more than 200 penalties.