Tesco has said that around 600,000 Tesco Clubcard accounts may have been subject to fraudulent activity.
Tesco says that password and username combinations may have been stolen from other websites and used to try to access Tesco accounts where customers used the same username and password.
Tesco said that no one will lose the value of any of their Clubcard vouchers or points, but it will be issuing new cards and vouchers. It says it is contacting affected customers and telling them to reset their passwords.
A Tesco spokesperson said: “We are aware of some fraudulent activity around the redemption of a small proportion of our customers’ Clubcard vouchers.
“We have strict security measures in place and our priority is protecting our customers. Our internal systems picked this up quickly and we immediately took steps to protect our customers and restrict access to their accounts. At no point was any customer’s financial data accessed.
“We have asked customers affected to reset their passwords and are contacting customers whose Clubcard vouchers may have been affected to let them know that we will replace these vouchers and issue new Clubcards as a precaution. We are sorry for any inconvenience this may cause.”