The Lloyds group of companies, including Bank of Scotland and Lloyds TSB Bank, received more than 200 complaints per day in the first half of 2011, according to data published by the Financial Ombudsman Service (FOS).
The group, which also includes Scottish Widows and Black Horse, received a total of 37,696 complaints in the six months to 30 June.
Lloyds TSB bank received almost 20,000 complaints, with almost 17,000 related to the alleged mis-selling of payment protection insurance (PPI). More than 80% of those were resolved in favour of the consumer.
Of advisory businesses, Towry received the most complaints (140), with more than 90% found in favour of the complainant.
In the previous complaints period - June to December last year - Towry received 345 investment-related complaints, with 83% resolved by the FOS in the clients' favour.
Overall, the service said it received 149,925 new complaints in the first half of the year - or more than 800 per day. This was 54% higher in number than the previous six months.
Two-thirds of the new complaints were about payment protection insurance (PPI).
During most of the period, four of the largest high street banks put complaints about PPI mis-selling on hold pending the outcome of a test case on the issue.
That case went against the banks in April, when they were ordered to clear the backlog complaints by the end of August, either by paying compensation or rejecting a complaint.
The FOS said a decline in the number of complaints it upheld in the period to 47% from 53% in the preceding six months reflected the uncertainty caused by the PPI legal review as it made cases harder to resolve.
Smoking biggest culprit; obesity second
Average earner will gain £840 in 2018
Will also move heritage items
Responding to letter from Treasury Committee chair Nicky Morgan