Drivers could face a £100 charge for parking outside a supermarket's opening hours.
Some supermarket car parks restrict when vehicles can be left there, even if the car park is empty.
Select Car Leasing highlighted the issue.
Graham Conway, managing director of Select Car Leasing, urged drivers to read signs before deciding whether to park.
He said: “Check the signage as you enter the car park carefully before making a decision about whether to stay or not.
“Not all store car parks punish after or before-hours parking, but many do, and it’s an issue that’s causing real confusion among motorists.”
Mr Conway said the company had seen cases where people were charged after leaving a vehicle for a matter of minutes.
Supermarket car parks can be managed by private operators, which issue Parking Charge Notices when they believe conditions have been broken.
Some use automatic number plate recognition cameras, with notices sent by post.
Other conditions can include limits on the length of a stay, parking within marked bays and restrictions on who can use disabled or parent and child spaces.
A private Parking Charge Notice is different from a penalty charge notice issued by a council.
In England and Wales, Citizens Advice says drivers should check whether the parking company belongs to an accredited trade association when considering an appeal.
It advises that if a non-member puts a ticket on a vehicle, the driver should not contact the company unless it writes to them first.
In Scotland, Citizens Advice says someone who wants to challenge a private ticket should first contact the operator, explain their reasons and ask for it to be withdrawn.
It says they could also approach the landowner, such as the supermarket, and provide the operator with evidence including photographs of unclear signs.
In Northern Ireland, the Consumer Council says a driver who believes a private notice was issued incorrectly may be able to appeal, using evidence such as photographs of damaged signs.
It says registered keepers in Northern Ireland cannot automatically be held liable for private parking charges, unlike in England and Wales, and that the driver is liable.
An independent appeal may be possible if an operator rejects a challenge, depending on the company and the relevant appeal service.
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