More than £ 8million will be refunded to customers after HSBC and Santander broke overdraft rules.
The Competitions and Markets Authority said both banks agreed to refunds for actions that took place in February 2018, after new rules came into force.
HSBC was found to have broken the rules twice twice, while Santander broke them on six occasions.
Refunds will cover all fees incurred by customers from going into unarranged overdrafts where they had not been warned ahead of time by the text alerts.
Some 115, 000 HSBC customers are in line for refunds, while Santander has not yet said how many people are due money back.
HSBC broke the rules because it had a policy of not disturbing customers after 10. 45 pm, the CMA found.
This meant that, if customers went into an unarranged overdraft between 10. 45 PM and 11. 45 pm, when balances were calculated, they were not alerted by text.
Most were told the next day, after fees had already been charged.
An HSBC spokesman told Mirror Money:“ Having been the first bank to auto-enrol customers to unarranged overdraft SMS alerts, and seeing that six in ten of our customers who receive one then pay in money avoiding additional charges, we appreciate how helpful these text messages can be. We apologize to those customers who for different reasons did not receive an alert.
“We will continue contacting customers who incurred overdraft charges as a result of these issues to apologize and provide a refund.”
Santander had missed sending texts to customers who had in the past signed up for email alerts, and also failed to update phone numbers properly, along with a litany of other failures.
A Santander spokesperson said: “We are very sorry that some customers in certain circumstances were not sent the required overdraft alerts. The introduction of these alerts is a move we welcomed and believe is a real support to customers.
“We have carried out a detailed review to understand why the errors happened and have taken steps to fix the issues. We are now working to identify and refund all affected customers as quickly as possible. ”