Video report by ITV News Business Editor Joel Hills
Tesco Bank customers will not be able to complete transactions online throughout Monday after a hacking attack.
The bank's chief executive Benny Higgins said the move was an emergency security measure.
Mr Higgins did not say if the online transactions freeze would last beyond Monday.
He added that, of the bank's 136,000 current account holders, 40,000 had seen "suspicious transactions" with 20,000 having had money taken.
Some current accounts were subjected to "online criminal activity" over the weekend, with "some cases" resulting in money being withdrawn fraudulently, he said.
Mr Higgins said customers affected by the block will still be able to withdraw cash and use other services.
He also sought to reassure customers that Tesco Bank will "ensure they do not lose out financially" and accounts will be refunded "as soon as possible".
The Financial Conduct Authority says banks must refund unauthorised payments immediately, unless they have evidence that the customer was at fault or the payment was more than 13 months ago.
The bank, which has 7.8 million customer accounts, is working with authorities and regulators to address the circumstances surrounding the security breach.
Tesco Bank told ITV News that customers who are concerned that their account may have been affected can contact them on 0345 835 3353 and via Twitter @tescobankhelp.
The bank has also contacted current account customers by text message to notify them of the issue.