Lloyds Bank was the target of a cyber attack which prevented a number of customers from using their online accounts.
The breakdown in service from the group, which includes Halifax and Bank of Scotland, came after the websites were overwhelmed with millions of requests in a denial of service attack.
The attack lasted three days earlier this month. The National Cyber Security Centre (NCSC) is said to be investigating the attack with Lloyds.
Lloyds said it was convinced the “vast majority” of users were able to gain access to their accounts and move money around as normal during the attack.
The bank stated: “We experienced intermittent service issues with internet banking between Wednesday morning and Friday afternoon the week before last and are sorry for any inconvenience caused.
“We had a normal service in place for the vast majority of this period and only a small number of customers experienced problems.
“In most cases, if customers attempted another log in, they were able to access their accounts. We will not speculate on the cause of these intermittent issues.”
The value of fraud committed in the UK last year rose to a record value of £1.1 billion on the back of a huge increase in cyber crime.