Tesco is issuing new cards to 600,000 Clubcard account holders after discovering a potential data breach.
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.
Tesco said no financial details have been compromised, and that its systems have not been hacked - adding that the new cards are merely a precautionary measure.
"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 that new vouchers would also be issued.
The UK loyalty scheme offers one point for every pound spent in store. Every 100 points are worth £1.
In November, Tesco unveiled a controversial new Clubcard with a "£400 a year" saving.
The card, which has now launched, will give customers additional discounts on groceries, F&F, Tesco Mobile, as well pet food, homeware and toys.
However it'll cost you £7.99 a month - or an eye-watering £95.88 a year.
Benefits include 10% off two big shops worth up to £200 each, every month, 10% off F&F, Fred & Flo, Go Cook, Fox & Ivy, Tesco Pet and Carousel all year round, and double data for new and existing Tesco Mobile customers.
Shoppers will also be able to apply for a Tesco Bank credit card with no foreign exchange fees later this year.
More recently, the grocer issued a warning to shoppers over forgotten loyalty points.
It said the equivalent of 650,000 weekly food shops is currently unclaimed.
These points can also be tripled when used with one of Tesco's 150 Reward Partners, such as Thorpe Park or Pizza Express - which could save families a fortune over half term.
