Shoppers who use store cards to pay for Christmas will be forced to repay billions of pounds in “extortionate” interest payments.

A new study shows that the total bill for storecard holders who use the cards for their Christmas shopping will be £2.8 billion next year - 62 per cent more than the interest paid by credit card customers.

Most major retailers, including TopShop, Argos, B&Q and Selfridges, offer store cards. But the rates of interest are often prohibitive. The average rate is 26 per cent, while some cards levy 30.9 per cent interest, more than double the average credit card rate.

Consumer groups have slammed store cards for piling more pressure on already over-indebted consumers. Paul Davies, head of money research at Which?, the consumer group said shoppers should simply “cut the cards up.”

But despite this, the number of cards has more than doubled in the last five years as shops push the cards at their tills. Nearly 15 million storecards have now been issued.

Sales assistants are instructed to encourage customers to take the cards, highlighting the free perks and interest free period which can last for up to two months.

But shoppers who make a purchase on the card but fail to clear their balance before the interest-free period ends are hit by the punitive interest charges.

A Playstation 3 bought on an Argos storecard costs nearly double the £350 retail price if a shopper makes only the minimum payment on the card each month, according to uSwitch, a financial website.

Mr Davies said: “Sales staff in stores are incentivised to promote store cards and we are concerned that people won’t realise that the rate they have to pay is really very high. A monthly rate of 2.2 per cent may seem manageable, but the annual percentage rate (APR) is actually 29 per cent, which is extortionate.”

Some retailers, including Burton and Marks & Spencer, have actually raised the rates on their store cards since last Christmas by an average of 2.7 percentage points.

One sales assistant told The Times: “We are constantly reminded that it is part of our job and are expected to ask every customer if he or she would like a store card... I am well aware that most of the customers have no idea what APR means.”

The Competition Commission found last year that store card lenders were charging customers £55 million too much every year, but experts say the investigation had little impact.

The average interest rate charged by store card providers has decreased by just 0.3 percentage points since the investigation. Vince Cable, acting leaders of the Liberal Democrats, said that retailers should cut the cost of their cards or be fined. He said: “It has already been demonstrated by the Competition Commission that store cards are too expensive. If big fines can be imposed on supermarkets and airlines for price fixing, then we should get get equally tough with lenders pushing exorbitantly priced cards which push people deeper into debt.”

Earlier this year, the Competition Commission ordered all store card providers which charge an APR of more than 25 per cent to publish a ‘health warning’ on the card.

Miss Selfridge, Warehouse and BHS all charge 29 per cent interest on their cards, while Topshop charges a more modest 19.9 per cent. LaserUK, which operates store cards for shops including JJB sports and Faith, charges 30.9 per cent on some cards.

The Finance & Leasing Association (FLA), which represents store cards lenders, said that store cards were good value and suggested they could help those who had problems with their credit history. Stephen Sklaroff. FLA Director General, said:

“For many consumers, store cards represent good value, and provide a variety of benefits. For some people a store card can be a good way of starting a credit history of responsible money management.”

Mike Naylor, of uSwitch, said: “Consumers really need to wake up to this money trap.”