Tesco Bank has halted online banking transactions after fraudsters stole money from 20,000 customer accounts.
The banking division of Britain's biggest supermarket chain said 40,000 accounts had been affected by "online criminal activity" over the weekend, but money was fraudulently taken from only half of the accounts.
Tesco (TESO) said it was suspending online transactions for current (checking) accounts as a precautionary measure.
The bank did not say how much money was stolen in total. But customers using Tesco's online support forum were reporting that sums of up to "several thousands" of pounds had vanished from their accounts.
Tesco Bank said it was hoping to refund all customers within 24 hours.
Some customers were having troubles reaching Tesco Bank, and complained about the supermarket giant "not answering phones" on social media.
"I have had two texts and emails to call fraud team. Was on hold for over 45 minutes and got cut off," one customer wrote on Twitter.
Another customer complained in Tesco's online help forum: "Spoke to Tesco after 1 hour 20 minutes on hold, like others, just waiting for a call back and no sign of my £2,400 today. I'm taking the day off work, I can't go in feeling as low as this."
The bank has just under 8 million customers and roughly £8.1 billion ($10 billion) in saving deposits.
