Barclays has been named as the most complained about financial services firm between January and June, and upheld 62 per cent against it over the same period.
The bank was also the most complained about firm in the second half of last year, with an uphold rate of 44 per cent.
Data published by the FCA today show Barclays received 370,733 new complaints in the first half of 2013, down 12 per cent from 414,302 on the previous six months.
Overall there were 2.9 million complaints made between January and June, a fall of 17 per cent from 3.4 million between July and December.
The figures relate to firms who have reported over 500 new complaints within a six month period.
The Financial Ombudsman Service publishes separate data about the complaints it receives where the initial complaint has been rejected by the business.
Lloyds TSB was the second most complained about financial services firm with 253,735 complaints, a fall of 38 per cent from 349,386 in the previous six months. The bank upheld 62 per cent of complaints against it over the same period.
MBNA, Lloyds subsidiary Bank of Scotland, and Santander made up the top five most complained about firms, with uphold rates of 36 per cent, 45 per cent and 43 per cent respectively.
Complaints about missold payment protection accounted for 1.8 million, or 61 per cent, of new complaints.
PPI accounted for £2.4bn of the total £2.55bn paid out in redress to customers in the first half of 2013.
Investment complaints accounted for £54m of redress, while banking complaints represented £52m of redress.
Lloyds TSB received the most investment complaints between January and June, a total of 3,092 and up 54 per cent from 2,009 in the second half of last year.
FCA chief executive Martin Wheatley says: “We expect firms to put their customers at the heart of their business – an important part of this is the way they handle customer complaints.
“Publishing complaints data is a powerful tool that helps encourage competition between firms to improve their service to customers, and helps consumers assess their relationships with banks and other providers.”