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Borrowers could be caught out by fees on balance transfer credit card deals after being enticed by 0% interest offers, a consumer group has warned.

These deals allow people to put old debts onto a new card.

Which? estimates that consumers pay about £334m in fees to do so each year, but it said few understood the costs involved.

In its survey, it said seven out of 10 people incorrectly thought the transfer was free.

Which? argued that the City watchdog should rule on whether the fee should be shown as a monetary sum rather than a percentage, or even consider banning firms from advertising deals as 0% when there was a fee.

It said that many people were blinded by the interest rates on this debt, so failed to realise that some deals were more expensive owing to the fees that were charged.

It also pointed out that these products were best for those who wanted to pay off credit card debt, and not those who needed the credit card for further spending.

"Too many credit card deals appear to include sneaky fees designed to catch customers out," said Richard Lloyd, Which? executive director.

"With millions now using credit cards to pay for essentials, it's vital that the [City watchdog] Financial Conduct Authority takes action to ensure consumers are well protected.

"We want the regulator to scrutinise balance transfer deals and make it easier for people to understand their true cost."

The UK Cards Association, which represents the card providers, said many deals provided good value and that customers should spend time choosing the appropriate product.

"The information on fees and charges which must be provided when applying for a credit card is prescribed by law," said Richard Koch, head of policy at The UK Cards Association.

"In addition, card issuers provide a summary box setting out all the costs associated with the credit card, which is included in all marketing and pre-contract materials, so that customers can easily compare different products. Card companies must also notify customers before a deal comes to an end."