Up to 447,936 customers of Lloyds Banking Group were affected by a data breach caused by an IT glitch that briefly exposed transaction details to other users, according to a letter published by the Treasury Select Committee.

The incident, which affected Lloyds, Halifax and Bank of Scotland customers on 12 March, saw some users shown other people’s transactions while using online banking services.

According to the letter, 114,182 people clicked on transactions that were not their own and may then have been shown more detailed personal information, including account details, national insurance numbers and payment references.

The Treasury Committee said the issue was caused by a software defect during an overnight update.

Lloyds Banking Group has so far paid £139,000 in compensation to 3,625 customers for distress and inconvenience. No customers have yet been identified as having suffered financial loss.

The letter also acknowledged that transaction information relating to individuals who are not Lloyds Banking Group customers may have been visible during the incident.

The development follows questions put to the bank last week by Dame Meg Hillier, chair of the Treasury Committee, after the outage affected online services.

Hillier has asked Lloyds Banking Group to provide further updates in one month and again in six months.

The committee noted that, in March last year, it found nine of the UK’s biggest banks had amassed at least 33 days of IT outages over the previous two years.

Hillier said: “Modern banking methods mean we can now perform a variety of tasks on our phones in a matter of seconds, and almost anywhere.

“What this incident brings into focus is the fact that there is a trade-off. 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.”