Shoppers with store cards are paying an average of 62% more interest than those using credit cards, research suggests.
Credit card customers may not realise that they face higher fees
Some 16 million Britons owe a total of almost £2.2bn on store cards, according to comparison website Uswitch.com.
It said the average interest charged on such cards was 26%, with some charging as much as 30% - nearly double the average credit card rate of 16%.
Since May, store cards charging more than 25% must carry "wealth warnings" that cheaper deals may be available.
Uswitch personal finance expert Mike Naylor said: "People are typically lured into store card deals at the checkout because they will get a small discount on the purchase being made.
"In most cases, this discount will be completely negated by the high levels of interest incurred by those who do not pay the balance off in full each month.
"More importantly, a busy checkout in a department store is not the best location to allow people to make an informed financial decision and read the small print before signing up."
The introduction of store card "wealth warnings" earlier this year followed an investigation into the sector by the Competition Commission, which found retailers and card providers were protected from competitive pressures and so were able to charge consumers a high repayment rate.