Barclays is to refund more than 300,000 personal loan customers because of errors it made on their loan statements.
Affected customers will see the interest they paid returned and the bank has set aside at least £100 million to compensate them.
From next month the bank will contact customers it believes received mistaken statements from October 2008, when changes to the Consumer Credit Act came in. The errors on statements stem from a failure to fully comply with these rule changes.
Barclays said it is also looking at whether the mistakes could have been repeated across other parts of the business, including Barclaycard.
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In a statement, a Barclays spokesperson 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."
It is not known exactly how much affected customers will receive in refunds.