Thousands of Nationwide customers will be getting a refund after the building society failed to warn them they would be charged for entering an overdraft.
The Competition and Markets Authority (CMA) says customers must be warned via text message before they are charged for an unarranged overdraft.
But the watchdog ruled that although Nationwide did send out text messages to customer, it failed to inform them of the charges they faced.
Around 70,000 are thought to be affected by the error and they will now be refunded a total of £900,000 in charges.
People who are owed money are being contacted by Nationwide and refunded automatically.
It is the second time in six months that the CMA has taken action against Nationwide for breaking the order, having previously secured £6 million in refunds for customers in August 2019.
Adam Land, senior director of remedies, business and financial analysis at the CMA, said: "Banks and building societies that fail to send customers text alerts saying they will be charged if they enter an unarranged overdraft are breaking the rules.
"The fact that Nationwide is a repeat offender makes it even more serious.
"Following our action, it will now repay all affected customers, and quickly."
Nationwide has appointed an independent auditor to review its processes, as directed by the CMA, and has already begun refunding its customers.
Sara Bennison, chief marketing officer at Nationwide Building Society, said: "The CMA directions, issued to Nationwide in August 2019, required the society to complete an independent review of its processes in relation to text alerts.
"While all members received their texts on each and every occasion, this review identified that alerts sent to members who were in collections did not explicitly state that they would be charged an unarranged overdraft fee.
"While these members haven't been overcharged, we appreciate these texts are designed to help people avoid unarranged overdraft charges, so we apologise that on this occasion we didn't meet the high standards we set ourselves.
"We are contacting impacted members and will be automatically refunding the charges back into their account.
"From November 11 2019, the society removed unarranged overdraft charges, so this issue will not occur again in the future."
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