Banks are employing "underhand" tactics to deter customers who challenge them over overdraft charges, Which? has claimed.
The consumer campaign group said it had uncovered evidence of banks threatening to close accounts, passing details on to debt collectors and delaying and charging for statements as they seek to put off disgruntled customers.
The disclosure comes at a time when the Office of Fair Trading (OFT) is preparing a report on bank fees, with many predicting that financial institutions could be forced to cap the amount they charge for going over agreed overdraft limits.
Recent research suggested that some banks charge as much as £30 in penalties for a service that costs them just £4.50.
Doug Taylor, personal finance campaigner at Which?, said: "We believe that banks have been over-charging customers who exceed their overdraft limit for years, charging billions in the process.
"In an attempt to avoid paying consumers what they are due, we have found that banks are employing increasingly underhand methods to avoid their responsibility to treat their customers fairly and refund the charges."
A growing number of consumers are challenging banks over "unfair" penalty charges, with many prepared to take the matter to court.
But in its investigation Which? found evidence of banks threatening to close the accounts of people who had claimed against them.
One customer found that not only had his account been closed while his court claim was ongoing, but that the bank in question had instructed debt collectors to act on the account.
Responding to Which?'s report, Angela Knight, chief executive designate of the British Bankers' Association, said: "Which? is clearly trying to exploit its position as a consumer body by sensationalising what could be a useful piece of research. The banking industry handles over seven billion transactions a year and occasionally something will go wrong - that's human nature."
