In April 2009, Clydesdale, owned by National Australia Bank, discovered an error in how it had calculated mortgage repayments for customers with variable rate mortgages.

As a result, incorrect repayments were made on over 42,500 customer accounts. Of these, approximately 22,000 accounts were left with shortfalls, because customers made repayments that were insufficient to repay their mortgages by the end of the agreed terms.

These 22,000 customers then faced unexpected increases in their monthly repayments both to correct the error and to make up for their shortfalls.

In total there was a £21.2 million shortfall in Clydesdale mortgages, with customers who underpaid left with mortgage balances higher than they should have been.

According to the FCA, the shortfalls range from under £20 to over £18,000, with an average of £970.

After discovering the error, Clydesdale contacted customers and set up a dedicated call centre to deal with any queries.

However, in seeking repayment from customers as a priority, it wrongly sought to balance its own commercial interests against the requirement to treat customers fairly.

Letters the bank sent to customers suggested that they had no alternative but to bring their repayments up to date.

Many customers, however, could have rejected demands to repay the shortfalls caused by Clydesdale’s calculation errors. This lack of clarity was compounded by poor instructions to Clydesdale’s call handlers for dealing with customers who called to complain.

In response, David Thorburn, Chief Executive of Clydesdale Bank, said: “I am very sorry that this wasn't handled as it should have been. We should have made it clear at the time that this was entirely our fault and that some customers may be entitled to compensation.

“Our priority is to fix this for customers as quickly as possible and they will each receive a letter explaining how we will make this right for them. In addition, an automatic compensation process for customers who underpaid as a result of our error is in place to provide immediate resolution for over 14,000 customer accounts. Those customers will receive a full apology and confirmation of their refund within the next 48 hours. We will work with the remaining customers, whose cases are more complex, to discuss how they may have been affected and what their options are. The vast majority will hear from us by mid-October.

“We have voluntarily agreed to take action that will go over and above that required by the FCA. While the fine has been calculated under the new and more stringent penalty regime, we are grateful that our efforts to put this right for customers quickly have been recognised and the penalty reduced.”

Speaking on the fine, Tracey McDermott, the FCA’s director of enforcement and financial crime said of the fine: “For most people mortgage payments are their biggest monthly outgoing and we all budget on the assumption that the information our mortgage lender gives us about what we need to pay is correct.

“Here Clydesdale failed in that basic duty and, when it discovered the problem, sought to pass all of the consequences on to its customers - expecting them to find the money to remedy mistakes which were entirely of Clydesdale's making.

“Firms must put the interests of customers at the heart of their business if we are to restore trust and confidence in financial services. Clydesdale is today paying the price for its decision to put its bottom line ahead of the need to ensure its customers were treated fairly."