More complaints were made about Barclays than any other banking brand by UK customers in the first half of the year, figures have shown.
The bank received 251,563 complaints, with 53% of closed cases upheld in customers' favour, the Financial Services Authority (FSA) figures show.
Barclays said it had cut complaints by 14% compared with a year earlier.
Other brands high on the list included Lloyds TSB (181,907), Santander (168,888) and NatWest (147,109).
The data pulls together figures released in recent weeks by banks.
Nearly 10,000 complaints were filed every day to financial institutions, with a total of 1.85 million made in the first six months of the year.
The FSA figures showed that, among the most complained-about banking brands, Santander was the most likely of the major brands to deal with cases within eight weeks.
It closed 98% of cases within that timeframe. This compared with 74% at Royal Bank of Scotland, 77% at Lloyds TSB, 86% at NatWest, 89% at Barclays and 90% at HSBC.
Complaints were dominated by those about payment protection insurance (PPI), especially after banks lost their legal challenge on PPI rules in April.
PPI is supposed to cover borrowers' loan repayments if they fall ill, die, or lose their jobs.
But mis-selling cases led to new rules on how cases should be dealt with, and also created an extra compensation bill running into billions of pounds for the banks.
Adam Scorer, of watchdog Consumer Focus, said: "This issue continues to dog the financial sector and is a big test of its commitment to treating consumers fairly.
"All firms need to deal with outstanding cases and make sure everyone affected is treated efficiently and fairly."
Complaints about banking, rather than insurance and some other categories, fell by 22% compared with the same period a year earlier.
The FSA's complaints figures are published relating to banking brands.
Barclays headed the list but said the number of complaints had fallen by 14% compared with the same period a year earlier.
"We want to get it right every time. When we do get it wrong, we apologise, try to correct it quickly and identify how to prevent it from reoccurring," said Antony Jenkins, chief executive of Barclays Retail and Business Banking.
"We have made good progress in reducing complaints with a substantial and sustainable reduction in banking complaints by nearly a third.
"However, there is much more to be done and we are working hard to further improve our service to our customers, putting them at the heart of our business and getting it right first time, every time."
The largest group - Lloyds Banking Group - had most complaints when all its brands were added together.
Some complaints that are unresolved by the banks themselves end up with the independent Financial Ombudsman Service. It recently said that the largest number of these complaints, in the first half of the year, also related to Lloyds Banking Group.
It also said that nearly two-thirds of the new complaints made in the six months to the end of June were about PPI.