Tesco blocks 620,000 Clubcard accounts amid security issue
The supermarket said someone has stolen password and username combinations from other websites and used them to try to access Tesco sites.
Tesco said its systems had not been hacked and no financial data was accessed.
However, as a precaution it is cancelling and replacing all affected vouchers and reissuing physical cards.
Tesco said customers will not lose the value of any of their Clubcard vouchers or points.
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 are sorry for any inconvenience this may cause.”
Tesco added customers should reset their passwords as a precaution.