Lloyds bank credit card customers are being urged to check their bank statements after numerous reports of fraudulent activity.
Rob Burgess, founder of flyer and loyalty credit card website Head for Points, said dozens of his readers saw fraudulent transactions appear on their Lloyds Bank Avios Amex cards.
Customers who have a Lloyds Avios Rewards credit card have been urged to check their statements.
A number of the transactions have appeared from shops in the US.
Mr Burgess said: "Many readers had fraudulent transactions from shops in the US charged to their Lloyds Avios cards.
"It's quite serious as these are not online transactions, but real transactions in physical shops.
"This means real plastic cards were made to match the number of their cards."
A spokeswoman for Lloyds Bank admitted that "a very small number" of Lloyds Bank Avios Rewards American Express credit card customers have been affected by recent fraudulent activity.
She said: "This has affected less than one per cent of customers who hold these cards and we have introduced additional controls to provide further protection.
"These controls have been successful in ensuring that fraudulent transactions are identified and declined."
The Lloyds Bank Avios Rewards credit card account comes with an American Express and MasterCard card to earn Avios (air miles for BA and Iberia) with.