Barclays have described the paperwork blunder which will cost the bank £100 million next month as "technical documentary errors".
The error means the bank will have to pay 300,000 customers who have taken loans with the bank a refund of several hundreds of pounds each.
In a document Barclays confirmed it had identified "issues" with information contained in statements and arrears notices relating to customer loan accounts.
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.