There was a big surge in complaints to the Financial Ombudsman Service (FOS) about payment protection insurance (PPI) during the last part of 2011.
According to new data seen by BBC News, there were a total of 55,907 complaints in the final quarter of last year, up by ten per cent from the three months before.
This was due to a 57 per cent increase in PPI complaints, with 30,301 received about this issue alone.
Other problems with products such as credit cards, mortgages and bank accounts made up only 18 per cent of the complaints.
It was found that 68 per cent of the PPI cases saw the FOS rule in customers' favour, up from 66 per cent the previous year.
Banks have already been forced to pay out millions of pounds in compensation after mis-selling PPI to people who did not need it or would not be able to claim on it in the event that they fell ill or lost their jobs.
The large institutions have set aside more than £6 billion to cover the cost of payouts to customers who have still not had their cases resolved.
Earlier this month, it was revealed by the Financial Services Authority that around £160 million in compensation was paid by banks to customers last year.
Barclays was the worst offender, while HSBC was also criticised for its poor practices.
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