Tesco Bank have suspended a number of bank accounts after more than 20,000 customers had money stolen after a cyber attack.
The amount of money stolen has not been revealed but the attack is being treated as a "criminal investigation".
Customers have seen online transactions freeze and cards blocked as part of emergency security measures.
Tesco chief executive Benny Higgins told Press Association: "Tesco Bank can confirm that, over the weekend, some of its customer current accounts have been subject to online criminal activity, in some cases resulting in money being withdrawn fraudulently.
"We apologise for the worry and inconvenience that this has caused for customers, and can only stress that we are taking every step to protect our customers' accounts.
"We can reassure customers that any financial loss as a result of this activity will be resolved fully by Tesco Bank, and we are working to refund accounts that have been subject to fraud as soon as possible."
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