Interest rates quoted by credit card companies can often mislead people, consumer group Which? has said.
Which? said the 14 different methods used by lenders to calculate interest charges make it hard for consumers to tell which credit card is cheapest.
The consumer group called for a single method of calculation to be adopted.
"It's ludicrous that a card with a lower interest rate can cost more than one with a higher rate," Martin Coles, editor of Which? magazine, said.
As an example of the confusing picture facing consumers, Which? calculated it was possible for someone borrowing on a credit card charging a rate of 11.8% to pay more in interest than through a card charging 13.9%.
Last year, the industry started incorporating a so-called "honesty box" in their credit card statements which outline the costs of the loan and any additional charges.
Investigation
On Thursday, the Office of Fair Trading (OFT) announced that it is to investigate the fees charged by Mastercard to retailers.
The OFT says that interchange fees - the fee charged between banks for processing a card transaction - are too high.
The fees, it said, meant that consumers ultimately paid more for goods.
The OFT announced a similiar investigation into Visa's interchange fee last October.
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