Credit card companies make £240 million from 'interest free' offers

Many who switched card companies to save money on their interest payments were unaware they would be charged up to three per cent of the balance for making the change.

That means someone moving a £5,000 debt had to pay £150, while a £3,000 transfer cost £90.

The Office of Fair Trading is considering a probe into the fees – just days after it launched a High Court action against banks on the issue of overdraft charges.

Zero per cent balance transfer deals have been widely used by savvy consumers – or "rate tarts" as the banks call them – to manage their debts.

There are almost 150 cards which do not charge interest, but almost all now charge a fee for making the transfer.

Of the few cards with no fee, including Norwich and Peterborough Building Society, and Britannia Building Society, the deal is for members only.

The fees became widespread last year as banks and building societies sought a new source of income after the OFT cracked down on penalty fees.

More than £239million was paid in transfer fees in the year to the end of June, according to MoneyExpert.com.

Almost 12million people have switched credit cards in the past year, with an average fee of £19.99.

Aspokesman for the British Bankers' Association said the fees covered the transfer of direct debits and checking credit history.

However, why this charge would be higher for those transferring bigger sums was not clear.