British cyber security authorities are investigating the theft of money from thousands of Tesco Bank accounts as experts warned that other small banks could also be vulnerable to attack.
The National Cyber Security Centre (NCSC), a new government body, said on Tuesday that it was working with criminal investigators and Tesco to understand the nature of an attack described as "unprecedented" by the financial regulator.
The NCSC and Britain's National Crime Agency said they could not think of another confirmed case where thieves had stolen large sums of money via a mass hacking of accounts at a Western bank.
Tesco Bank, owned by retailer Tesco, halted online transactions from all current accounts on Monday and said it would repay people who had lost money in an attack that targeted some 40,000 accounts, nearly a third of its 136,000 accounts.
The bank has provided few details about what happened.
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Cyber security experts said that smaller banks were attractive targets for hackers because they were not as well protected as big global financial institutions, some of which spend hundreds of millions of dollars a year on cyber security.
Britain's financial regulator sought to reassure the public that financial authorities were working to understand the nature of the attack.
"This looks unprecedented in the UK," Andrew Bailey, chief executive of the Financial Conduct Authority (FCA), told a parliamentary hearing.
He said customers should be compensated by the end of the day but that it would be difficult to restore full online services at Tesco Bank until the cause of the breach was fully understood.
Reported attacks on financial institutions in Britain have risen from just five in 2014 to more than 75 so far this year, according to FCA data, but bank executives and providers of security systems say many attacks go unreported.
