Car rental firm Europcar is accused of operating illegally in the UK for two years, it has been reported.
Europcar is accused of selling rental agreements for terms of longer than 84 days when it did not have a licence to do so, the Telegraph reported.
Such a contract requires an agreement with the Financial Conduct Authority (FCA) which it did not have between July 2015 and June 2017.
Europcar staff and management at broker rentalcars.com say that it continued to rent vehicles for periods of longer than 84 days, a claim that the company has not denied.
James Daley, director at consumer campaign group Fairer Finance, said that it could mean affected customers could get some or all of their money back but the FCA was also partly responsible for not spotting the discrepancy.
"You don't expect a large well known high street brand like Europcar to be flouting the law. It is their responsibility to be setting the example and leading the way," he told the Telegraph.
"If it was licensed before 2015 then I would assume it was aware of the rules. It speaks of the kind of company culture which breeds a cavalier regard for the law."
A Europcar spokesman said: "As part of the normal course of its business, Europcar UK works with all authorities including the FCA in a fully co-operative and transparent manner."
A spokesman at Rentalcars.com said: "Europcar made us aware of this issue on the 5th of July 2017. As a result, we ceased taking any bookings for Europcar over 84 days in length.
It is the latest potential scandal for the firm which already faces claims it overcharged half a million customers for repairs sparking an inquiry by the Serious Fraud Office (SFO).
If the SFO finds Europcar guilty of fraud over the claims it overcharged customers for repairs as revealed earlier this year, it may be fined up to 10% of its turnover.