Tesco Bank has said that £2.5m had been stolen from 9,000 customers over the weekend in what cyber experts said was the first mass hacking of accounts at a western bank.
Tesco Bank said it had resumed full service after the theft, which forced the suspension of online transactions on Monday.
"We’ve now refunded all customer accounts affected by fraud and lifted the suspension of online debit transactions so that customers can use their accounts as normal," Tesco Bank CEO Benny Higgins said in a statement.
The bank, whose operating income has accounted for as much as a quarter of Tesco's total in some years, added that no customer data had been compromised.
The National Cyber Security Centre, a new UK government body, said 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 remember another confirmed case where thieves had stolen large sums of money via a mass hacking of accounts at a Western bank.
The bank has provided few details about what happened.
It is not clear how online thieves broke into the bank, how they pulled out the funds or how much was stolen. It is also not clear if there are any suspects.
A spokeswoman for Tesco declined to comment beyond its previous statement on Monday.
