There are calls to tackle banking charges after fines totalling £3 billion were handed out for unauthorised overdrafts last year.
Which? is urging banks to abolish these charges, which it feels unfairly sting far too many customers.
A new survey conducted by Which? revealed that one in four people used an unauthorised overdraft in the last 12 months and almost two thirds of these people had done so more than once.
"Unauthorised overdrafts are a gold mine for banks - the charges they levy are way out of proportion," says Malcolm Coles, editor of Which?
"It's especially unfair on customers who only occasionally run up a small unauthorised overdraft for a short time."
With so many using the service it suggests it is not just a case of people mis-managing their finances - those who make a genuine mistake are being heavily penalised as well.
For example, an unauthorised overdraft of £50 for three days would cost you over £95 at Natwest - the bank with the highest unauthorised overdraft rate of 33.78 per cent.
Which? would like to see all banks follow the lead of Alliance & Leicester and HSBC, which now charge the same rate for unauthorised overdrafts as they do for authorised overdrafts - Alliance & Leicester charges 6.90 per cent and HSBC charges 14.80 per cent.
Mr Coles concluded by saying: "We're calling on banks to stop punishing these customers by giving them a few days' grace to pay back an unauthorised overdraft before charges are levied."
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