Tesco Bank has admitted having to block some customers' credit cards after "suspicious activity" on thousands of accounts was detected by its internal fraud prevention system, with frustrated customers now claiming their balances have been raided by fraudsters.

Tesco Bank, which issued a statement on its website and company Twitter account, said the number of people impacted is "in the thousands, but less than 10,000" and urged anyone impacted to get in contact about any ongoing problems.

However, outraged customers instead took to social media to complain about poor communication with the service and lengthy waiting times when calling. Some customers claim to have already moved their money out of Tesco Bank "as a precaution".

In a statement published on 6 November (Sunday), Tesco Bank apologised to customers "for the inconvenience" and said "we want to do everything we can to protect our customers from fraud".

It added: "Yesterday our fraud prevention systems identified suspicious activity on a number of accounts. As a precautionary measure, we have blocked some customer cards and have contacted affected customers via text message to notify them."

Yet some customers 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.

A woman called Kirsty Brown claimed to be in a similar situation. "We have been hacked, all money gone, no email or text! Appalling response from Tesco so far," she tweeted. Another commenter, Sam Allen,

Another commenter, Sam Allen, said: "This has left me unable to feed my kids in school tomorrow."

Speaking to the BBC, the Financial Conduct Authority (FCA) said that banks in this situation must refund all unauthorised payments immediately, unless they can produce evidence the customer was at fault or the payment was more than 13 months old.

In a statement to IBTimes UK, a spokesperson said: "We have taken steps to protect a small proportion of customers' current accounts from fraud following an alert yesterday.

"All our customers can continue to access their account as normal through online banking and can make chip-and-pin transactions. For those impacted, we are working to re-establish a full banking service and we apologise for any inconvenience."

On the Tesco Bank support page, a slew of customers have voiced their outrage at the situation. "I will be moving my money, not because I am worried about fraud but because the response to this incident has been appalling and shows complete contempt for customers," wrote one commenter.

Another said: "We can't get through because Tesco won't give out any information through their website or social media channels [and] thousands of customers are trying to call. If they were more honest with customers about what was going on, people wouldn't need to call and the phone lines would not be blocked."

Have you been affected by the issues? Get in contact: j.murdock@ibtimes.co.uk or via @Jason_A_Murdock