A new list of banks facing the highest number of complaints from UK customers is topped by Lloyds Banking Group.
The state-backed group, which includes a number of different brands, received more than 280,000 gripes in the first half of 2010, the City regulator said.
However, Lloyds said it received the most because it was the largest bank.
Santander was the worst of the major High Street banks at dealing with gripes within eight weeks, said the Financial Services Authority (FSA).
The watchdog is also proposing new rules to improve banks' complaints procedures.
In April, the FSA criticised the way banks dealt with customers' complaints.
It blamed a lack of interest by senior bank management, bonus schemes that inhibited staff from paying compensation, and poor decision making, but said that banks had agreed to make big improvements.
It prompted some banks to publish their complaints statistics over the summer, and these have fed into the list compiled and now published by the FSA.
Among the major banks, the list of banking complaints is topped by Lloyds, which received 288,717 complaints in the first six months of the year.
This is followed by Barclays, with 259,266, and Santander with 244,978.
Lloyds said that on average, the complaints came from less than 1% of their 30 million customers.
"Our relationship with our customers is at the heart of our business and we take all feedback very seriously. Like every organisation we know there are areas where we can improve and we are working with our customers to do just that," said a Lloyds spokeswoman.
"The vast majority are happy with the service we provide and this is reflected in the low number of complaints we receive relative to the high number of accounts our customers hold."
Among the data published by the FSA is the proportion of complaints dealt with within eight weeks.
Here, Lloyds fared much better, with a completion rate of 97% for Lloyds TSB customers. Barclays dealt with 91% of cases within this timeframe.
However, Santander only closed 46% of complaints within eight weeks.
Steve Williams, director of service quality and complaints at Santander, said: "We have introduced a number of initiatives to help improve service across the bank, including investing in 1,000 new jobs across our busiest branches and call centres, with the aim of reducing queuing at our busiest times and improving the quality of service that can be given to each customer.
"Improving service quality remains a priority for Santander."
The FSA, which has published the full list for the first time, has also proposed a series of changes to the way complaints are dealt with.
- stopping banks sending letters which reject complaints but fail to explain that customers can challenge this and go to the Financial Ombudsman Service
- stipulating that banks identify a senior manager responsible for complaints handling
- putting in remedies for common complaints
- raising the maximum compensation level that can be ordered by the ombudsman from £100,000 to £150,000.
"Good complaints handling standards should be the rule not the exception," said Sheila Nicoll, the FSA's director of conduct policy.
The banking industry trade body said that the latest data from the FSA could be confused with other figures published by the Financial Ombudsman, and could be taken out of context.
"The UK banking industry is committed to its customers and to resolving problems as quickly as possible, that is why a lot of complaints are resolved by close of business the next day," a spokesman for the British Bankers' Association said.
"It is extremely important to keep the figures published by the FSA in context: the larger the bank is the more complaints it is statistically likely to receive.
"With more than 140m bank accounts in the UK and billions of transactions a year there will inevitably be instances when things go wrong.
"The banking industry welcomes greater transparency but is concerned that the separate publication of complaints data by the Ombudsman and the regulator could lead to data overload. What should be a useful overall summary could become a complex and confusing exercise."
"We will also be responding in full to the FSA's consultation on changing the complaints handling regime. We agree on the need for a clear, transparent complaints process which sets out all the options for customers."