Lloyds Banking Group received nearly 2,000 complaints a day from disgruntled customers during the first half of the year, figures showed today.
The part-nationalised bank received a total of 288,717 complaints during the six months to the end of June - the equivalent of 1,850 each working day.
But it closed more than three times as many complaints as it received at 715,458, after the end of a test case on unauthorised overdraft charges enabled it to deal with complaints on the issue that had been put on hold.
Lloyds TSB received the highest number of complaints for any brand in the group, with 103,686 people contacting it over banking problems during the six months, while it also closed 348,700 cases relating to disputes on the issue.
But the group only upheld 12% of complaints on banking, although it pointed out that the figure was distorted by the high number of cases relating to unauthorised overdraft charges, which were rejected after the banks won the test case.
The bank also received 41,874 complaints about insurance products during the first half of the year, mainly relating to the mis-selling of controversial payment protection insurance.
Customers complaining about insurance were successful in 54% of cases dealt with during the period.
Bank of Scotland, which includes the Halifax, Intelligent Finance and Birmingham Midshires brands, received 68,280 complaints about banking during the first half and closed 246,733.
But it found in favour of only 7% of customers, although this again reflected the fact that many of complaints related to unauthorised overdraft charges.
The Bank of Scotland arm of the business also received a high number of complaints about insurance at 36,121, more than two-thirds of which were upheld, while 9,071 people complained about home finance products, such as mortgages, 37% of whom were successful.
A Lloyds Banking Group spokeswoman said: "Our relationship with our customers is at the heart of our business and we take all feedback very seriously.
"Like every organisation we know there are areas where we can improve and we're working with our customers to do just that."
But she added that the complaints figures should be seen in the context of the fact that the group has more than 30 million customers, the highest number in the UK, and less than 1% of these people had complained to it during the first half of the year.
The Financial Services Authority has given firms until the end of this month to publish data on the number of complaints they receive.
Nationwide has also published figures showing it opened 90,208 complaints during the six months to April 4, more than 84,000 of which related to banking.
But when complaints relating to unauthorised overdraft charges are stripped out the group received 28,620 complaints.
It upheld only 16% of all banking complaints it looked at during the period, but found in favour of 58% of customers once the unauthorised overdraft complaints were excluded.