281,000 complaints were made to Barclays in the second half of 2011, as the bank topped the complaints table published by the FSA.
This compares to 251,000 for the first half of the year.
Meanwhile, Lloyds Banking Group saw just under 241,000 complaints, up from 182,000 from January to June 2011.
Which? executive director, Richard Lloyd, said: “Today’s complaints data is evidence that some banks are still failing to treat their customers fairly when things go wrong. The increase in PPI complaints further demonstrates just how widespread PPI mis-selling was.