The top 20 card providers charge interest in 12 different ways, says the consumer organisation Which?
Credit card interest 'confusion'
Which? says the top 20 providers charge interest in 12 different ways, meaning a "lower" Annual Percentage Rate (APR) can be dearer than a "higher" one.
The APR is supposed to make it easier for consumers to compare credit deals.
Which? wants the OFT to standardise the system so that people can compare deals more accurately.
It found quoted rates of APR can vary according to whether interest is charged on interest, and whether there is an interest-free period.
Interest can also be charged from when the transaction is made, or when the transaction reaches the account.
The organisation is one of a handful of consumer groups which can make a "supercomplaint", which then forces the OFT to investigate.