At least 300,000 of Barclays Bank's personal loan customers are in line to be refunded for mistakes the bank made on their paperwork.
Lending regulation states that if errors are made in loan paperwork, then all interest paid by borrowers must be returned to them.
It is estimated that the bank's bill for the interest refund payments will reach £100 million, with affected customers expected to receive a few hundred pounds each, depending on how much they borrowed.
The mistakes date back to October 2008, and relate to customers' arrears notices and statements, which a Barclays spokesperson referred to as "technical documentary errors".
In a statement, the bank said "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 said all affected customers will be contacted as soon as possible. The bank also said it is now investigating whether similar mistakes had been made in Barclaycard customer paperwork.
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