Tesco Bank has been forced to block some customers' cards after thousands appeared to have been targetted by hackers.
Thousands of accounts were reportedly affected after apparent fraudsters targeted the bank's clients, with customers taking to social media to alert the bank.
One man tweeted his available balance had dropped by £700 without him making a transaction while another said the disruption had left her "unable to feed my kids in school tomorrow".
Others complained about a lack of communication from the bank and hours spent on hold.
"We have been hacked, all money gone, no email or text! Appalling response from Tesco so far £nobodyanswering," one tweeted.
Addressing the issue on its website, Tesco Bank said it wanted to do "everything we can to protect our customers from fraud" and urged those who feared their accounts had been hacked to contact them immediately.
The statement, issued on Sunday morning, said: "Yesterday our fraud prevention systems identified suspicious activity on a number of accounts.
"As a precautionary measure we have blocked some customer cards and have contacted affected customers via text message to notify them."
Tesco Bank, which opened in 1997 and has been wholly owned by Tesco PLC since 2008, has 7.8 million customer accounts.
