Barclays Bank may have to pay out up to £100 million to around 300,000 of its personal loan customers after making mistakes on their paperwork.
Up to 300,000 customers could be in line for a few hundred pounds from the issue which dates back to October 2008.
A spokesman described the problem, which relates to arrears notices and statements, as "technical documentary errors" .
It was revealed in a 185-page bank document in which Barclays stated it has " identified certain issues with the information contained in historic statements and arrears notices relating to consumer loan accounts. It is therefore implementing a plan to return interest incorrectly charged to customers".
A Barclays spokesman said: "Barclays has proactively reviewed information it has historically sent to its customers relating to interest charges where we have found technical documentary errors.
"As a result Barclays has identified certain issues with the information contained in some statements and arrears notices relating to consumer loan accounts.
"Due to these notification errors, interest was not due on certain accounts during the period that Barclays made this mistake, and whilst no one has been mis-sold to, customers are entitled to have their interest payments returned. No customer will pay more than they were ever contractually expected to.
"Barclays has notified the Office of Fair Trading (OFT) which is responsible for consumer credit issues and is implementing a plan to return interest payments to customers as swiftly and efficiently as possible.
"Barclays is undertaking a review of all its businesses where similar issues could arise to assess any related issues.
"Any affected customer will be contacted by Barclays and customers do not need to take any action."