Complaints to the Ombudsman (FOS) about financial institutions increased by 15% in the last six months last year compared with the first half of 2010.
There were 97,237 new complaints in total.
Lloyds Banking Group topped the list of the most complained about institution with a staggering 22,181 complaints filed – accounting for more than one in five of all the complaints received by the FOS.
The high street banks dominated the top five spots with more than 6,000 complaints each referred to the FOS, accounting for over half of all the new complaints received.
Second was Santander racking up 6,759, followed by Bank of Scotland with 6,743. HSBC and Barclays also received 6,736 and 6,535 complaints respectively.
The majority of complaints were about mis-selling of payment protection insurance, relating to over half of all new cases referred.
On average, 53% of complaints were upheld in favour of customers - a rise from 44% in the first six month of 2010.
Why don't more people complain?
Natalie Ceeney, chief executive and chief ombudsman at the FOS, says: "The latest set of complaints data continues to show that while some financial businesses are improving the way they handle their customers' complaints, some regrettably are not."
"The FOS is committed to publishing information and sharing insights from the tens of thousands of cases we see – so that businesses, regulators and stakeholders can all learn both from where things have gone wrong, and from where problems have been put right to everyone's benefit" she adds.
Commenting on the result, Martin Dodd, customer services director for the Lloyds Banking Group, says: "Our customers tell us the two areas we need to improve are the length of time it takes to resolve complaints and the perception that the bank is not taking sufficient ownership of them. We are committed to getting this right."
The data is from 1 July to 31 December 2010 and relates to consumer complaints about individual financial businesses, including banks, insurance companies and investment firms.