Payment protection insurance (PPI) made up 56% of the total complaints to the financial ombudsman between April and June.
Despite the high proportion of PPI complaints, the total amount actually dropped from the previous quarter from 76,522 down to 57,790 complaints.
The ombudsman found in favour of the complainants in a half of all new cases in Q2.
Current accounts and credit card accounts were the second largest complaint taking 6% of complaints each according to the latest figures.
UKCG stopped selling PPI in 2010 and the financial product has been seen as problematic for dealers.