Santander is refunding about £1.4 million of charges levied on customers after imposing unarranged overdraft fees on about 20,000 people with current accounts without first sending them an alert.
Those customers affected hold either an Everyday current account or an Essentials current account.
The Competition and Markets Authority (CMA) wrote to Santander about breaching the Retail Banking Order.
Part of the order aims to ensure that current account customers should receive an alert about unarranged overdraft charges before they incur them.
The majority of affected customers have now been reimbursed.
A Santander spokeswoman said: “We are sorry that some customers did not receive unarranged overdraft alerts and we are in the process of refunding customers who incurred unarranged overdraft charges as a result.
“We notified the Competition and Markets Authority when we became aware of the issue and continue to keep the CMA updated on progress.”
Customers who opened either an Everyday current account or an Essentials current account between February 1 2018 and February 20 2019, and who went into unarranged overdraft, did not receive an alert warning them of the possibility of charges.
Customers went on to receive charges for exceeding a pre-agreed credit limit.
Santander has put procedures in place to prevent any further breaches.