A quarter of UK bank customers have experienced problems with their account in the past 12 months, according to a survey by consumer group Which?.
The survey found the most common problems were poor customer service, the bank trying to sell inappropriate products or difficulty in reaching an adviser, the Telegraph reports.
A fifth of customers who complained to their bank said the problem was not resolved satisfactorily.
Lloyds TSB had the highest proportion of customers experiencing problems in the past year, followed by the Co-operative Bank and Bank of Scotland.
First Direct and Nationwide customers were the least likely to experience problems.
Richard Lloyd, executive director at Which? said: "When things go wrong it is critical that banks act swiftly and fairly to deal with the problem, identify what caused it and make sure it's not repeated.
"Some banks are getting this right but they all need to do a lot more to put their customers first."
Customers of Lloyds TSB had particular problems changing their address or personal information, while mistakes on statements was a top issue for customers of the Co-operative Bank.
Poor customer service was the principle problem for Bank of Scotland customers, while Barclays had the highest proportion of customers reporting problems with incorrect charges.
A spokeswoman for Lloyds Banking Group, which includes Lloyds TSB and Bank of Scotland, said they were "disappointed" by the results and said "commitment to service is central to our aim of being the best bank for our customers".
Of the 12 banks surveyed, Barclays was the only one that offered a free 0800 complaints number for customers.
The survey of 2,029 UK adults in February this year looked at current account problems and complaints made to 12 banks and building societies: Lloyds TSB, Co-operative Bank, Bank of Scotland, RBS, Barclays, Santander, Halifax, NatWest, HSBC, Yorkshire Bank, Nationwide and First Direct.
A Co-operative Bank spokeswoman said: "We are genuinely disappointed with some of the findings in this survey, which do not reflect our experience with either customer satisfaction surveys or the number of complaints we receive.
"We are now working with Which? to understand the research in more detail and will be addressing those areas in which we have fallen short as a priority."
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