Tesco Bank has notified thousands of customers that they may have been targeted by fraudsters.
The supermarket-owned bank said it has identified "suspicious activity" via its fraud prevention systems on Saturday and is taking action as a precautionary measure.
"We are sorry for the inconvenience we know this causes, however we want to do everything we can to protect our customers from fraud. If you think you have been the victim of fraud, please contact us immediately," the bank said in a post on its community forum.
The bank said the number of customers thought to be affected is in the thousands, but less than 10,000 and is investigating the issue.
Many customers have reported being on hold with the bank for hours after being told to call its fraud unit via a text message in the early hours of Sunday morning.
Some customers have also had their cards blocked.
A Tesco Bank spokesperson said:
"We have taken steps to protect a small proportion of customers’ current accounts from fraud following an alert yesterday. All our customers can continue to access their account as normal through online banking and can make chip and pin transactions. For those impacted, we are working to re-establish a full banking service and we apologise for any inconvenience."