Choosing the cheapest credit card if you don’t pay your balance off each month is not as simple as looking for the one with the lowest headline annual percentage rate.

According to a shock report by the consumer lobby group Which?, there are now 14 methods of calculating interest due on credit card purchases, and consumers are finding it impossible to determine which card is genuinely a “best buy” or suited to their needs.

Which? says that if you have debt outstanding on your card, but pay your next bill in full, some companies charge interest up to the date of the bill, while others charge interest up to the point your payment is received.

So, if you did not pay your bill in full the previous month, you will be carrying more interest forward from month to month. You will also find yourself paying variable amounts of interest on that interest, and the only way to stop this is to overpay one month – although there is no way of calculating how much the amount due might be, so you could end up in credit.

Most card companies start charging interest from the date of your purchase, though some charge from the date the credit card company pays the retailer, which is a few days later.

APACS, the body that represents the credit card industry, and runs the website www.choosingandusing.com to help consumers select the most appropriate card, claims the way companies charge interest is a “competition issue”.

Which? says that this is nonsense, as most consumers don’t realise that you can’t compare cards simply by looking at advertised rates. And, even if they did, they still couldn’t work out which card is cheapest.