Current account customers are being charged as much as £900 a year for going overdrawn without permission, according to a report that "shatters the myth" of free banking.
Other ways in which banks make money from supposedly free accounts is by charging up to 19.9% interest for agreed overdrafts - more than many credit cards and personal loans - and through "hefty fees" for overseas transactions, said consumer champion Which?.
The findings are the latest blow for the beleaguered sector already reeling from a series of scandals and prompted Which? to call for more transparency around bank charges to help customers choose the right account.
Which? chief executive Peter Vicary-Smith said: "When some people are paying up to £900 a year in bank charges it completely shatters the myth that banking is free."
But the British Bankers' Association (BBA) branded the report "disingenuous" and said customers can still get free banking for accessing cash and making most types of transactions in the UK.
It added that consumers would expect to pay to go overdrawn because this amounts to borrowing money, while overseas charges were often beyond banks' control.
Which? said charges for unauthorised overdrafts vary greatly between banks.
The Halifax Reward Current Account charges £120 a year for going into the red for two days in a row every month, whereas at the other end of the scale the Yorkshire/Clydesdale Bank Current Account Plus charges £900 a year for the same scenario.
And many banks, including RBS/NatWest and HSBC, charge an annual percentage rate of 19.9% for authorised overdrafts.
More than six in 10 people surveyed said they had paid a charge they considered unfair, hidden or disproportionate.
And 94% thought banks should be more transparent about their charges.
But Which? rejected calls for all banks to charge for current accounts as "worrying".