Part-nationalised Lloyds Banking Group accounted for more than a quarter of all complaints made against financial services firms during the first half of the year, figures showed.
The Financial Ombudsman Service said it received 22,420 complaints about the group, in which the Government holds a 43% stake, during the six months to the end of June.
These accounted for 27% of the 84,212 complaints the ombudsman received for all financial services firms during the period.
Barclays was the second most complained about group, with the ombudsman receiving 9,215 complaints about the bank, followed by Ulster Bank parent Royal Bank of Scotland which has also received a Government bailout, at 6,469.
Spanish banking giant Santander and HSBC completed the top five most complained about firms at 5,372 and 4,031 respectively.
These five groups accounted for 47,507 cases referred to the ombudsman - more than half of all complaints received.
Within the total for Lloyds Banking Group, Lloyds TSB accounted for 12,750 complaints, while Bank of Scotland, which includes the Halifax brand, made up 6,211 of the cases.
But Lloyds Banking Group said the number of complaints it had received was low in relation to the number of customers it has, as the UK's largest bank.
A spokeswoman said: "The vast majority of our customers are happy with the service that we provide and this is reflected in the low number of complaints we receive relative to the high number of accounts our customers hold."
People complaining about the banks were most likely to have a dispute about banking and credit products, or general insurance ones, including controversial payment protection insurance.
Complaints against credit card firms were also high, with the ombudsman receiving 1,976 complaints about MBNA and 1,662 about Capital One.
The proportion of complaints upheld by the ombudsman fell slightly during the six months, dropping to 44%, down from 53% during the second half of 2009.
But the level of complaint found in favour of the consumer varied from just 14% for Clerical Medical Investment Group, which is part of Lloyds, to 100% for Eisis, which sells insurance.