The Office of Fair Trading (OFT) has been urged to continue fighting to help people reclaim unauthorised overdraft charges.
Consumer website MoneySavingExpert.com said it had taken legal advice that suggested customers could challenge the fairness of the charges under the Consumer Credit Act.
The website said that, under the Act, it was up to the banks to prove that the charges were fair, rather than for consumers to prove they were the opposite - shifting the burden of proof on to the banks.
But the law only applies to new charges from April 6 2007 and any charges before this date are included only if borrowing was still outstanding 12 months later.
MoneySavingExpert outlined other legal routes which could possibly be used to people challenge the fairness of the charges, but it called on the OFT to take up the fight for consumers.
The website's founder Martin Lewis said: "We believe there are substantial legal avenues available for it to challenge fairness - the OFT cannot give up."
Customers who incur an unauthorised overdraft or breach their agreed limit can be charged as much as £35.
This could include processing a bounced transaction, when the actual cost to the banks is as little as £2.50.
But in a recent test case the Supreme Court ruled that unauthorised overdraft charges were not subject to regulation by the OFT under unfair contract rules, dashing the hopes of millions who had tried to claim refunds.
The watchdog is currently considering the details of the judgement before it makes a decision on whether to continue its investigation into the charges.
