Individual firms will publish their own complaint details from next year
More than nine million individual complaints were made to firms in the financial services industry in the 2006-2008 period, figures have shown.
Banking and loans accounted for more than half the formal complaints made to financial companies in that period.
The Financial Services Authority (FSA) has published the complaints figures for the first time.
During the second half of 2008, 40% were being settled in the customers' favour.
The figures have been driven by huge numbers of complaints about bank overdraft charges, and the mis-selling of payment protection insurance and mortgage endowments.
"Publishing this information will mean that consumers and firms can now see how many complaints the industry receives and how it deals with them," said FSA director Dan Waters.
Individual companies will start to publish their own complaints data from July 2010.
The total number of complaints rose from 2,727,000 in 2006 to 3,411,000 in 2007 before dropping back last year to 2,903,000.
There would have been more but for the fact that the FSA told regulated firms in the middle of 2007 they they did not have to include new complaints about charges for unauthorised overdrafts levied by banks and building societies.
More than 1.2 million of these complaints have since been "parked" until the issue is settled by the courts.
"It's a poor reflection on the industry that there are so many unhappy customers out there," said Which? personal finance campaigner Phil Jones.
"Financial firms simply aren't treating consumers well enough and things must change if the industry is to rebuild its reputation," he added.
The FSA's figures were dominated by complaints about current accounts, especially by demands for the return of overdraft fees, which reached 3,513,000 over the three-year period.
"The spike in the total number of complaints in the first half of 2007 is the result of a large increase in complaints about overcharging and poor customer service in relation to banking and loans products at that time," said the FSA.
There were 908,000 complaints about the mis-selling of mortgage endowments during the period, although their number tailed off dramatically during the three years.
Complaints about credit cards came a close third at 745,000.
But a surge in complaints about payment protection insurance (PPI) saw the number of complaints about general insurance and "pure protection" policies double, from 62,000 to 127,000, between the first half of 2006 and the second half of 2008.
In April this year, the Financial Services Ombudsman (FOS), which deals with complaints that firms cannot settle themselves, accused many firms of being deliberately unhelpful.
However, a spokeswoman for the British Bankers' Association welcomed the publication of the data, saying that the vast majority of bank customers had "no problems" with their accounts or bank services.
"Millions of transactions for millions of customers go through the banking system every day," she said.
"Put in context, the proportion of reportable complaints is still very small at 3.5 per thousand products held," she added.
The vast majority of industry complaints were settled within eight weeks with, by the end of last year, just 10% taking longer to deal with.
Last year, the firms most likely to admit making a mistake were building societies, who settled 59% of complaints in their customers' favour.
Simon Morris, of City law firm CMS Cameron McKenna, was more critical of the FSA's publication of the data.
"Presenting crude statistics about complaints that firms receive provides no useful information to help investors make important decisions," he said.
"Instead it creates an alarming and inaccurate impression that firms are not to be trusted. It is irresponsible for the FSA to be undermining consumer confidence in such as insidious manner."