Consumers still find it very hard to compare credit cards as the way interest is calculated by each provider is not clear and that 12 of the top 20 card companies apply it in different ways.

Consumer magazine Which? said a “lower” annual percentage rate (APR) can be more expensive than a “dearer” one and that consumers were baffled by the different ways interest is calculated and charged.

Which? has demanded that the Office of Fair Trading (OFT) looks into the issue and that a standardised system is introduced.

“People simply cannot compare credit cards realistically as there are no fixed points to compare,” said retired financial adviser Peter Stanway.

“It has been calculated that if all card users had the most favourable calculation applied to their borrowing then they would save in the region of £400m a year in interest payments.

“It’s certainly not right that people cannot even usefully compare credit cards at the moment and, for that reason alone, I support the demand made to OFT for a standardised system as it would definitely make things fairer.”

However, the card issuers’ trade body, Apacs, said that consumer choice would be eroded if the system was standardised.

A spokesperson said: “There are a huge variety of cards on the market and some people prefer to have a lower APR but pay earlier, while others might like a slightly higher APR but only want to pay interest on the amount left outstanding.”