Complaints against high street banks have risen by almost half in the last year, according to the Banking Codes Standards Board, the banking watchdog.
Customer dissatisfaction appears to be rising
The sharp rise in complaints comes weeks after five of the biggest banks announced combined annual profits of £32bn - more than Luxembourg's GDP.
The largest number of complaints was about interest rates and charges.
It is now up to the city's regulator, the Financial Services Authority, to decide whether to take action.
It has a wide range of powers, including the ability to issue fines.
BBC business reporter Nick Cosgrove said the Banking Codes Standards Board is worried the latest rise in complaints could be the tip of the iceberg.
"It's not the only organisation to report a sharp rise in customer dissatisfaction with banks," he said.
"The Office of Fair Trading has announced a crackdown on what it calls unfair penalties on credit cards."