The Tesco Bank hack that saw money stolen from 20,000 accounts over the weekend was "unprecedented", the head of Britain's financial regulator has said.
The chief executive of the Financial Conduct Authority (FCA) said authorities were working to find the "root cause" of the breach.
"There are elements of this, as far as we can tell at the moment, that look unprecedented," Andrew Bailey told MPs at a Treasury Select Committee hearing.
Committee chairman and Tory MP Andrew Tyrie said the case looked "extremely serious", affecting around one in seven Tesco Bank account holders.
The bank has said it working to refund all affected customers after money was fraudulently withdrawn from 20,000 of its 136,000 current accounts over the weekend, with suspicious activity being tracked across 40,000 accounts.
A spokesman for Tesco Bank did not disclose the total amount stolen, adding that the incident is being treated as a "criminal investigation".
Online transactions have been temporarily frozen as part of emergency security measures, but customers affected by the block can still withdraw cash and use other services like chip and pin payments, while bill payments and direct debits "will continue as normal", Tesco Bank said.
The FCA boss said he was confident Tesco Bank can identify the affected accounts without customers needing to flag up discrepancies to the lender.
Tesco Bank assured customers that money would be reinstated by the end of Tuesday, but Mr Bailey said: "That's important but not by any means the heart of the concern.
"The heart of concern is what is the root cause of this and what it tells us about the broader threats."
The threat appears to have only affected the "debit card side of online banking", but Mr Bailey said "further urgent analysis" is required.