Thousands of people who took out personal loans with NatWest and sister bank Royal Bank of Scotland are receiving refunds on interest they were wrongly charged.
The banking giant is writing to tell those who are owed money about the mistake, which meant some customers falling behind with their payments should have been sent a "notice of sums in arrears", but this did not happen. It should not have charged interest to these borrowers until the correct notice was given.
The error occurred between 2008 and 2015. It is thought tens of thousands of people are affected but RBS did not give an exact figure.
The average payout being made is in the low hundreds, but the Daily Mail's Money Mail said in one letter it has seen, someone was refunded £1,130.75.
A spokesman for RBS said: "When we discover an issue we do everything we can to set it right.
"We didn't issue notices of sums in arrears correctly to a number of customers before we charged interest on their loans that had gone more than two payments into arrears. We are refunding these customers the interest we charged them at the time."