Tesco Bank has admitted suffering a breach that led to money being stolen from 20,000 customers' accounts.
The bank, which has more than seven million customers, confirmed on Saturday that around 40,000 accounts saw "suspicious transactions" over the weekend, of which half had money taken.
Benny Higgins, Tesco Bank CEO, said in a statement on the company's website: "Tesco Bank can confirm that, over the weekend, some of its customers’ current accounts have been subject to online criminal activity, in some cases resulting in money being withdrawn fraudulently."
Customers have reported that as much as £2,000 was siphoned from their accounts over the weekend. Many of those affected also complained that they were unable to get through to Tesco on the phone.
As well as suspending online transactions, Tesco Bank has blocked some customers' cards. The company said in a statement to the BBC that it hopes to refund customers within 24 hours.
"As a precautionary measure, we have taken the decision today to temporarily stop online transactions from current accounts. This will only affect current account customers," Higgins said.
"While online transactions will not be available, current account customers will still be able to use their cards for cash withdrawals, chip and PIN payments, and all existing bill payments and direct debits will continue as normal. We are working hard to resume normal service on current accounts as soon as possible."
However, the Financial Conduct Authority said that banks must refund unauthorised payments immediately, unless they have evidence that the customer was at fault or the payment was made more than 13 months ago.
We'll update this story as soon as we hear more.
Ultra-high-end all-in-one PCs from HP feature either 24-inch or 27-inch displays
Roomba 'smart' vacuum cleaner company iRobot plans to sell maps of users' homes to Apple, Amazon and Google
'Smart' products spying on their owners and selling the data for profit? Who'd have thought it!
TNT Express still struggling with NotPetya malware - crucial documents remain locked up in borked systems as staff grapple with manual procedures
TNT depots over-flowing with parcels as the company struggles to recover from NotPetya - while Reckitt Benckiser reports 'ongoing' recovery
Full roll-out of Android O expected within weeks