Britain's Tesco Bank said on Sunday that it had blocked some customers' bank cards after a fraud prevention system identified suspicious activity on a number of accounts.
"We have taken steps to protect a small proportion of customers' current accounts from fraud following an alert yesterday," a Tesco bank spokesperson said.
The bank, wholly owned by supermarket chain Tesco, says it has nearly 8 million customer accounts. It did not specify how many had been affected.
Affected customers would continue to be able to access their accounts online and make certain types of transaction using their cards, the bank said.