Tesco Bank has been thrown into chaos after 20,000 online accounts had money stolen in a hacking attack over the weekend.
The challenger bank (namely, not one of the Big Four), owned by the supermarket giant, has now frozen all online transactions in an emergency bid to sort out the situation.
Around 40,000 accounts were subjected to dubious activity, with half having cash removed.
Unprecedented in scale
Cyber security experts have said the incident may be unprecedented in terms of its scale but many of the precise details of what has happened are yet to emerge.
Tesco was among the big losers on FTSE 100 on Monday, with shares down over 1% to 200.4p.
In a statement the bank's chief executive Benny Higgins said: "....we have taken the decision today to temporarily stop online transactions from current accounts.
"This will only affect current account customers. 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."
He added: "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."
Jasper Lawler, an analyst at CMC Markets, noted that the Tesco bank debacle will not aid the Challenger banks' bid to get increasing numbers away from the big players like Barclays or Lloyds.
"The inability of many customers who lost money to reach online or telephone customer services after the fraud may cause some second-guessing over the switch to 100% digital banking," he said.
That said Aldermore shares (LON:ALD) added 2.76% on the day to 182.4p while Shawbrook (LON:SHAW) nudge 0.08% higher to 244.6p.
Customers to be refunded
Higgins has said he hoped customers would be refunded within 24 hours.
Tesco bank has over 7mln customer accounts and 4,000 staff.
Various customers' stories have been emerging, with one man saying he was left with around £21 after £600 was withdrawn fraudulently.
Another man has tweeted his balance had dropped by £700 without him making a transaction.
Many have complained at the bank's slowness in communicating the problem and people being left on hold for hours while trying to phone them.
Dept of Meaningless Statements: Tesco Bank boss says amount stolen frm 20,000 accounts is "a big number but not a huge number" #BBCR4today
— Ian Katz (@iankatz1000) November 7, 2016
The latest hack attack?
The latest is one of a series of high profile hacks on big companies in recent years.
In 2015, Talk Talk (LON:TALK), the mobile phone firm, saw almost 157,000 customers' personal details accessed when hackers targeted its web site.
In 2014, search engine giant Yahoo had the details of 500mln user accounts stolen.
Under rules from the regulator, the Financial Conduct Authority (FCA) banks must refund unauthorised payments immediately, unless the customer was at fault or the payment was more than 13 months ago. They must also refund any charges or interest added as a result of fraudulent payments.
Tesco Bank has been wholly owned by Tesco PLC since 2008. It started life as a joint venture with Royal Bank of Scotland.