Nearly half a million Lloyds Banking Group customers were impacted by a personal data glitch that allowed users to view other customers’ transactions, according to new details revealed to UK lawmakers.

Writing to MPs on the Commons Treasury Select Committee, Jasjyot Singh, CEO of consumer relationships for Lloyds Banking Group, said that while the incident was “fixed promptly”, up to 447,936 Lloyds, Halifax and Bank of Scotland customers may have seen other people’s transactions or had their data shared with other users. 

Singh’s letter goes on to say 114,182 people clicked on other people’s transactions when they became visible, and may have been shown more detailed information such as account details, national insurance numbers and payment references.

The letter also notes that transaction information related to individuals who are not Lloyds Banking Group customers may have been visible.

“Though this information should not have been visible, customers’ account balances were not affected, and customers were not able to perform unauthorised actions or move money on anyone else’s account,” confirmed Singh.

“Customers who experienced the issue would have been able to view others’ data momentarily, and the information that was visible would not be sufficient on its own for someone to carry out fraud against an individual’s bank account.”

In total, £139,000 of compensation has been paid out to 3,625 customers for distress and inconvenience, but no customers have, so far, been identified as suffering financial loss, with Singh stating that, in the bank’s assessment, it is “very unlikely” the information could be used to carry out fraud more widely. 

Singh’s letter identified the cause of the incident as a software defect within the mobile banking app’s API, which occurred during an overnight update, meaning that when a customer requested to view their current account transactions, their data was potentially visible to other customers who were, within small fractions of a second, requesting access to their own transactions.

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According to Lloyd’s data, the incident had a smaller impact than it might have done under a worst-case scenario, as 1.67 million customers were logged into the app while the incident was ongoing. 

“What this incident brings into focus is the fact that there is a trade-off,” said Dame Meg Hillier, Chair of the Treasury Committee.

“By moving more interactions with our bank online, we place our faith in technology, which can suffer unpredictable errors. It’s critical that consumers understand this, and that’s why my Committee continues to push banks to be transparent when things go wrong.”