By Julian Knight
Personal finance reporter, BBC News
About 1,000 people a day are phoning the Financial Ombudsman to complain about penalty charges levied by banks and building societies.
Customers are charged when they go overdrawn without permission
A year ago, the Ombudsman was receiving about 100 calls a week about the issue.
Over the past year consumer groups have been claiming that penalty charges are unlawful and should be refunded.
The Ombudsman said that in all penalty charge cases it had investigated, the bank or building society had settled before any judgement could be given.
The Ombudsman starts to investigate about 150 penalty charge cases a week, but the number of phone calls it receives from consumers looking to find out how to get penalty charges refunded is far greater.
In fact, the number of phone calls the Ombudsman receives on the penalty charge issue currently dwarfs any other area of complaint.
"Previously the largest number of complaints we received related to endowments," a spokeswoman said.
"However, we are getting at least three penalty charge calls for each one about endowments at present."
On Wednesday, following coverage of penalty charges on BBC TV's Watchdog programme, the Ombudsman received about 3,000 complaint calls.
At one stage the Ombudsman's website carried a message that its call centre was receiving higher than usual traffic.
Over the past year, the issue of consumers trying to reclaim their penalty charges has been gathering pace.
Last May, the Office of Fair Trading (OFT) said credit card default charges should only reflect company costs.
To the dismay of banks, the OFT added that this principle would apply to default charges on overdrafts.
At the same time, egged on by consumer groups such as Which? and the Consumer Action Group, an increasing number of people have been taking banks to the small claims court in a bid to get their charges refunded.
In response, banking groups have said that unauthorised overdraft fees are a charge for a service - and that in some European countries it is illegal for customers to go into the red without their bank's permission.