A data attack by fraudsters has affected 620,000 Tesco Clubcard holders.
Stolen password and username combinations were used in a bid to redeem reward vouchers that customers earn through the card’s loyalty scheme.
Holders hit in the scam told of their shock on social media.
Liam Wilson said: “Somebody has hacked my Tesco Clubcard points. It’s a mad world.”
Tesco confirmed that it had “become aware of fraudulent activity” around redemption of Clubcard vouchers.
But the supermarket giant stressed no personal financial data had been compromised.
A spokesman added: "We have strict security measures in place and our priority is protecting our customers," she added.
"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 believe that someone has stolen password/username combinations from other website(s) and used them to try to access Tesco sites - where customers used the same username and password.
"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."
