A condition on some credit cards means thousands of people making the minimum payment each month may never pay off their debt, according to Which?

With most cards, says the consumers association, the usual minimum payment is either a percentage of the balance (say 2 or 3 percent) or £5, whichever is greater. But for those with an MBNA card the minimum payment is calculated differently.

MBNA calculates the minimum payment as 2.25 percent of the balance, or the total of charges (interest, insurance, handling fees, etc) plus £5, or the balance if less than £5 - whichever is lower.

This means that for many people with MBNA cards, most of their monthly payment will simply cover charges, with only £5 going to clear their debt.

Malcolm Coles, Editor of Which? said: "Paying just the minimum each month is never a good idea, but the MBNA method of calculation simply means it takes even longer to reduce your debt so the credit card company gets paid interest for longer.”

"MBNA issues its own credit card and cards for Abbey, Alliance & Leicester and Virgin, as well as affinity cards. If you borrow on one of these and pay the minimum each month, check how your payment is calculated and think about switching to a card where your minimum payments will clear your debt faster.”

"You can also save even more on your interest bill by choosing a card that offers an interest rate of 0 precent for transferred balances for 6 or 8 months, but if it's going to take you much longer than that to pay off your debt go for a card that charges you a low rate on your transferred balance until you've paid it off."