Updated, November 9: Refunds totalling £2.5 million have now been paid out to 9,000 Tesco Bank customers, following a devastating attack on its systems that allowed fraudsters to steal money from thousands of current accounts.

Tesco Bank said that it had fully restored its service after temporarily freezing online transactions from current accounts.

Late on Tuesday, the company's boss Benny Higgins confirmed that it had refunded all customers affected by the fraud and claimed that "none of their personal data has been compromised" in the attack. He added:

Around 9,000 customers were affected by these fraudulent transactions and all customers affected have been fully reimbursed. We are continuing to work closely with the authorities and regulators in their criminal investigation of this incident. I’d like to thank our customers for their patience during this time, and to apologise for the worry and inconvenience this issue has caused.

Updated, November 8: Tesco Bank customers remain locked out from making current account transactions, two days after it was revealed that money had been stolen from 20,000 accounts over the weekend.

On Tuesday morning, the chief exec of the Financial Conduct Authority told parliamentarians sitting on the treasury committee that "I thought this looked unprecedented in the UK."

Andrew Bailey added that it was "too early to give a comprehensive account of what the root causes are."

It's now a race against time for Tesco Bank: it has until the end of Tuesday to reimburse its customers—some of whom say that hundreds of pounds was removed from their accounts.

The National Crime Agency is leading a criminal investigation into the attack on Tesco Bank's systems along with GCHQ's National Cyber Security Centre, which opened its doors last month.

Original story

Tesco Bank has been forced to suspend its online transactions after fraudulent criminal activity was spotted on thousands of its customer accounts over the weekend.

A total of 40,000 current accounts were hit by suspicious transactions. Money was pinched from 20,000 of the affected current accounts, Tesco Bank said on Monday morning.

"We apologise for the worry and inconvenience that this has caused for customers, and can only stress that we are taking every step to protect our customers’ accounts," said the bank's chief, Benny Higgins.

He added that Tesco was taking "a precautionary measure" by temporarily taking current account transactions offline. Higgins said:

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.

Tesco Bank has promised to refund any accounts affected by the fraud and added that it was working with police and regulators to help track down the malefactors behind the crime. The Financial Conduct Authority says it gives "higher priority to the protection of consumers as potential victims of fraud than to the protection of firms themselves as potential victims." Put another way, banks are expected to act swiftly when such fraudulent activity is detected.

Higgins told the BBC that the bank has around eight million customer accounts. He added that the number of customers hit by fraud was big but not huge. "It's 20,000 customers, we think it would be relatively small amounts that have come out but we're still working on that."

On Sunday, Tesco Bank said that it had "notified some customers that we have blocked their cards to protect their account."

However, some customers complained on social media about access to their current accounts being frozen without them first being informed of the fraudulent activity.

@tescobankhelp this has left me unable to feed my kids in school tomorrow can't put money on their dinner accounts can't buy sandwich stuff — SamAllenAVFC (@samallen72) November 6, 2016

my account @tescobankhelp @TescoBankNews has been hacked over the W/E & you text me at 7:21am today, but I cant log on to check how much ? — ? (@iWaveBack) November 7, 2016

Tesco Bank said it was trying to quickly refund all of the affected accounts, but it didn't reveal when the service would return to normal following the attack.