Customers at HSBC and Santander are to share a multimillion-pound windfall after the two banks were ordered to pay back overdraft fees.
The Competition and Markets Authority (CMA) said HSBC had failed to text 115,000 customers when they went into an unarranged overdraft, and would have to refund them £8 million.
Meanwhile, Santander also broke the rules in six different ways, more than HSBC's two breaches. It has agreed to pay back the customers that it did not text; however, it has yet to work out how many there are, or how much it owes them.
Customers will get back all the fees they racked up on their overdrafts, if they were not warned when going into the red.
The CMA said in a statement: "The refunds paid by the banks cover all fees incurred by customers from going into unarranged overdrafts where they had not been warned beforehand by the required text alerts."
The watchdog has also told both HSBC and Santander that they need to bring in outside help to review their compliance with the rules.
It said the two banks had started breaking the rule as soon as it came into force in February 2018.
The measure was part of a package brought in after an investigation into competition in the current account and small business banking markets.
HSBC broke the rules because it had a policy of not disturbing customers after 10.45pm, the CMA found.
This meant that, if customers went into an unarranged overdraft between 10.45pm and 11.45pm, when balances were calculated, they were not alerted by text.
Most were told the next day, after fees had already been charged.
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.
Santander 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."
HSBC said: "We appreciate how helpful these text messages can be. We apologise 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 apologise and provide a refund."
