Tesco is issuing new cards to 600,000 Clubcard account holders after unearthing a security issue.
The supermarket giant said it believed a database of stolen usernames and passwords from other platforms had been tried out on its websites, and may have worked in some cases.
No financial data was accessed and its systems have not been hacked, it added.
It said this was a precautionary measure and apologised for the inconvenience.
"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 said it had emailed everybody potentially affected, that nobody would lose their points and new vouchers would also be issued.
Tesco is issuing new cards to 600,000 Clubcard account holders after unearthing a security issue.
The supermarket giant said it believed a database of stolen usernames and passwords from other platforms had been tried out on its websites, and may have worked in some cases.
No financial data was accessed and its systems have not been hacked, it added.
It said this was a precautionary measure and apologised for the inconvenience.
"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 said it had emailed everybody potentially affected, that nobody would lose their points and new vouchers would also be issued.
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