Current account holders are being charged as much as £900 a year for going into their overdraft without permission, according to Which?.
The consumer champion conducted a report which "shatters the myth" of free banking, revealing customers face multiple charges on supposedly free accounts.
Banks also make money on such accounts 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 Which?.
According to the report, charges for unauthorised overdrafts vary greatly between banks.
For instance, the Halifax Reward Current Account charges £120 a year for going into the red for two days in a row every month. At the other end of the scale, the Yorkshire/Clydesdale Bank Current Account Plus charges £900 a year for the same scenario.
Which? chief executive Peter Vicary-Smith stated that paying large sums of money each year in banks charges "completely shatters the myth that banking is free", and called for more transparency to help customers choose the right account.
In addition, many banks, including RBS/NatWest and HSBC, charge an annual percentage rate of 19.9% for authorised overdrafts - hitting your pockets hard.
First Trust Bank's Classic Account charges £185 a year for going £200 into the red six days a month.
However, those of us that stay in credit can also be hit by charges for withdrawing or spending cash abroad. You can even lose out on accounts that do not give interest on savings.
As this is the case, why not check out the finer details of your current account. Don't like what you find, you can always switch!
Plus, if you are the sort of person who finds it hard to keep your finances in check, it may therefore be worth setting up an authorised overdraft facility just in case.
Alternatively, you could opt for a current account with a so-called overdraft buffer zone, which allows you to borrow a certain amount before charges kick in.
Getting back into the black
Using an overdraft can be an effective way to cover unexpected expenses, but it is not good way to borrow in the long term.
If you constantly find yourself in the red, or only have a positive balance for a few days of the month, it is therefore sensible to take steps to clear your overdraft.