The Competition and Markets Authority (CMA) said the bank had breached the Retail Banking Market Investigation Order between May 2021 and March 2022 by failing to send transaction histories to over 13,000 customers.

It follows a similar breach reported last year, prompting the regulator to impose legally binding directions requiring Monzo to make sure this doesn’t happen again, and to review its procedures with an independent body.

The directions are enforceable in court if Monzo fails to comply with them.

Adam Land, senior director at the CMA, said: “It’s simply not good enough for a major bank like Monzo to repeatedly fail its customers by not following clear rules.

“Having a record of your financial transactions can act as important evidence needed to secure a loan or mortgage – so Monzo’s failure to provide these put an unnecessary obstacle in the way of thousands of customers.

“We have ordered the bank to make changes which mean customers should not face this issue in the future. We’ll be watching to make sure proper procedure is followed.”

Monzo has now contacted all affected customers to offer them a copy of their transaction history.

It has also introduced new auto-alerts to warn staff when the system fails to issue transaction histories to customers and is monitoring cases to ensure departing customers receive their transaction history.

A Monzo spokesperson said: “Unfortunately, due to a processing error, some customers were not proactively issued with historic transaction data, however this information was always available to these customers at their request.

“As soon as we were aware of this issue, we fixed it on the same day and contacted all impacted customers as soon as possible. We apologise for any inconvenience caused.”

The CMA cannot currently impose financial penalties for breaches of this kind, but it has called for the power to do so.