Tesco Bank has admitted that "online criminal activity" discovered on tens of thousands of customer accounts resulted in unauthorised withdrawals and has now been forced to cease online transactions starting today (7 November).

In a statement posted online, Tesco Bank chief executive Benny Higgins said that current account customers will still be able to use their cards for cash withdrawals, chip and pin payments. All existing bill payments and direct debits will continue as normal, he added.

Previously, Tesco Bank blocked some customers' credit cards after "suspicious activity" was detected by its internal fraud prevention system on thousands of accounts, with frustrated customers claiming their balances had been raided by fraudsters.

Higgins told Radio 4's Today programme that roughly 40,000 accounts had suspicious transactions. While according to one BBC finance correspondent, Simon Gompertz, money was raided from roughly 20,000 customer accounts.

Higgins' statement continued: "Tesco Bank can confirm that, over the weekend, some of its customers' current accounts have been subject to online criminal activity, in some cases resulting in money being withdrawn fraudulently.

"This will only affect current account customers. We are working hard to resume normal service on current accounts as soon as possible. That is why, as a precautionary measure, we have taken the decision today to temporarily stop online transactions from current accounts.

"We apologise for the worry and inconvenience that this has caused for customers [...] we continue to work with the authorities and regulators to address the fraud and will keep our customers informed through regular updates on our website, twitter and direct communication."

On Sunday (6 November) the bank issued a statement via its website and company Twitter account that informed frustrated customers that the number of people impacted was "in the thousands but less than 10,000" and urged anyone impacted to get in contact about the ongoing problems.

As previously reported, the Financial Conduct Authority (FCA) told the BBC that, in this situation, banks must refund unauthorised payments immediately, unless they have evidence that the customer was at fault or the payment was more than 13 months ago.

On social media, customers have continued to voice their frustrations at the disturbances – with some already threatening to move their money to another service. Tesco Bank has not yet clarified if the fraudulent activity was the result of an orchestrated cybercrime attack.

Yet regardless of the cause, some customers have complained about significant financial losses related to the issues. One man, called Christopher Mills, said on Twitter his balance had been reduced by £700 without his authorisation and that he could not reach Tesco Bank representatives by phone.

Additionally, a woman called Kirsty Brown said: "We have been hacked, all money gone, no email or text! Appalling response from Tesco so far," she tweeted. Another commenter on Twitter, called Sam Allen, said: "This has left me unable to feed my kids in school tomorrow."

In his official statement, Higgins said: "We can reassure customers that anyfinancial 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."