LONDON (Reuters) - British digital bank Monzo has asked around half a million of its customers to change their card PIN (personal identification number) after discovering a security flaw in its system.

The fast-growing lender - which has 2.5 million users and recently launched in the United States - said the flaw meant some customers’ PIN codes had been visible within log files to engineers at Monzo.

In a blog post on Monday the bank said the flaw had been discovered on Friday and fixed in the early hours of Saturday, with the sensitive information deleted by Monday morning.

Monzo said it had emailed customers affected, advising them to change their PIN code at a cash machine.

The bank added that it had checked all the accounts affected and determined the information had not been used to commit fraud.

Monzo is one of Britain’s most-valuable start-ups, with a fundraising in June valuing the company at more than 2 billion pounds ($2.43 billion), according to the lender.