Some credit cards offering lower interest charges can work out more expensive than those with higher rates, a consumer watchdog warns.

Which? claims that many companies are concealing the real cost of their credit card from shoppers, making it difficult for people to secure the best deal.

It says that those who start charging interest from when the card is swiped at a retailers are more expensive than those who wait until they issue the payment.

This is not usually made clear to people signing up for a new credit card but the difference can make some credit cards up to 83 per cent more costly than others.

Which? found that cahoot charges an 11.8 per cent APR but because it starts charging interest as soon as the purchase has been made, this works out more expensive than using a HSBC credit card with a 13.9 per cent APR.

Credit cards from American Express, Cahoot, HBOS, MBNA, Mint, RBS/Natwest, Sainsbury's and Tesco were all found to conceal their charges but HSBC, Liverpool Vistoria and Saga were found to offer a better deal.

Which? editor Malcolm Coles said: "It's ludicrous that a card with a lower interest rate can cost more than one with a higher rate."