Tesco is issuing new cards to 600,000 Clubcard account holders after the supermarket discovered a security issue.
The step has been taken as a precautionary measure and the supermarket confirmed that no financial data was accessed and its systems have not been hacked.
The supermarket said that it believed a collection of data, including usernames and passwords, may have been stolen from another organisation and used in order to try and gain access to Clubcard accounts.
"We are aware of some fraudulent activity around the redemption of a small proportion of our customers' Clubcard vouchers," a Tesco spokesperson said.
"Our internal systems picked this up quickly and we immediately took steps to protect our customers and restrict access to their accounts."
The supermarket added it had emailed everybody potentially affected, that nobody would lose their points and new vouchers would also be issued.
A spokesperson for Action Fraud said: “Always use a strong, separate password to protect online accounts that hold your personal or financial details.
“You can use a password manager app, or the password manager built into your web browser, to securely store all of your passwords.
“Don’t forget to install the latest software updates so your devices and apps are protected against the latest threats.”