Thousands of Tesco Bank account holders have been targeted by fraudsters.
Customers have reported large sums of money vanishing from their accounts while others said their accounts had been blocked.
The bank has alerted some customers that their cards have been blocked as a “precautionary measure” after identifying “suspicious activity”.
Alan Baxter complained that £600 had disappeared from his account, leaving him with almost nothing left.
He tweeted: “£600 short here, £21 left. Going to Tesco to get shopping, will just walk out with it #seewhattheysay.”
Another customer, Nikki Jones added: “Read on twitter about fraud, checked balance, money gone, no communication from them.”
Tesco Bank, which is owned entirely by Tesco, said the number affected was “in the thousands, but less than 10,000.”
A spokesman told the Standard: “Yesterday our fraud prevention systems identified suspicious activity on a number of customer accounts.
“As a precautionary measure, these cards were immediately blocked to protect our customers and alerts sent to notify them.
“We are investigating this matter and if customers have any concerns we would encourage them to contact us directly.”
Tesco Bank has more than seven million customer accounts and employs 4,000 staff across offices in Edinburgh, Glasgow and Newcastle.