The competition watchdog has hit banks including Monzo and NatWest with a warning after they broke a rule requiring them to send historic bank statements to former customers. 

The Competition and Markets Authority (CMA) said that almost 150,000 people, mostly former Monzo customers, may have found it harder to borrow money or apply for a mortgage because they were not given their transaction history in the required time after switching banks. 

Monzo failed to write to more than 143,000 former customers, with NatWest, Bank of Ireland and Virgin Money accounting for the remaining breaches. 

The rules were introduced in 2017 to address concerns that customers who switch banks could lose access to their banking history – a key requirement for lenders offering credit. 

Under the rules customers must be sent their banking history within 40 days of closing their account, with at least 95pc sent within 10 working days.

The watchdog said it found some transaction histories were provided a few weeks late, while others were delayed by more than a year. 

"We will be watching closely to make sure these leading names stick to their word and don’t let their customers down again," warned Adam Land of the CMA.

"The Bank of Ireland, Monzo, Natwest Group, and Virgin Money should be in no doubt that the CMA stands ready to take further action if these failures are repeated." 

A Monzo spokesman said: "Since we identified the issue and reported it to the CMA in February 2021, we’ve fixed the problem and are telling affected customers that we can send them their transaction history if they’d like. 

"This was an admin error, and we’ve also said sorry to the customers involved. Up until now, we’ve sent a full transaction history to any customer who has asked for it directly."