The challenger informed the CMA it had failed to send transaction histories to more than 13,000 customers, despite reporting a similar breach last year.
Monzo has since contacted all affected customers to offer them a copy of their transaction history, the CMA says.
The CMA has issued 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, the CMA warns.
