The part state-owned institution and its subsidiaries accounted for more than 22,100 of the 97,000 complaints made by unhappy customers in the six months to December 31st.
Lloyds TSB attracted a total of 12,234 new complaints - the most of any single business.
Overall, the number of complaints to the FOS increased by 15 per cent in the second half of the year compared with the first six months of 2010.
The ombudsman upheld consumer grievances in 53 per cent of cases, up from 44 per cent during the previous six months.
"While some financial businesses are improving the way they handle their customers' complaints, some regrettably are not," said chief ombudsman Natalie Ceeney.
She added that the service is "committed to publishing information and sharing insights from the tens of thousands of cases we see".
Last week, Lloyds reported pre-tax profits of Â£2.2 billion ($3.5 billion) for 2010.
By Asim Shah