Two airlines have been accused of illegally charging credit and debit card fees to UK holidaymakers.

New rules, which came into force last month, ban credit and debit card surcharges for British customers.

But Dutch-based KLM and Air France were found to be illegally charging fees to credit and debit card users booking via their UK websites.

Both airlines blamed a banking mix-up for the issue.

A probe by consumer experts at Which? Money found the airlines may have been using flawed technology to incorrectly charge users a fee.

Which? investigated after a series of complaints from customers of Air France and KLM - now part of the airline group AFKLM - and found both were applying a two per cent fee for payments with some MasterCard and all American Express credit cards online.

From January 13 new EU rules banned retailers from charging customers a fee to use Visa and Mastercard credit or debit cards, with the exception of corporate cards.

The ban on surcharges has been widely adopted by government bodies and retailers, including airlines and travel companies.

UK legislation has gone further than other EU states, banning retailers from charging a fee on any 'payment instrument', including American Express and payment platforms like PayPal, although commercial cards are excluded. Air France and KLM are based in Europe, so do not have to comply with these more robust payment rules, despite marketing to UK consumers on a UK website.

Which? investigators booked flights on the UK websites for KLM and Air France using three personal credit cards and a debit card.

The payment pages for both airlines gave the option to pay by PayPal, bank transfer, credit or debit card. The option to pay by credit card warned that a fee may apply.

Air France charged fees of up £2.13 for a flight from London to Paris, while KLM charged £16.27 for a trip from London to Mauritius.

Which? Money said these rogue charges are probably not limited to Air France and KLM.

It found both airlines were using payment technology that mistakenly classes personal credit cards as business ones, which can be charged a fee.

Gareth Shaw, Which? Money expert said: 'The ban on surcharges should stop consumers being penalised simply for using their card.

'However, we have found ways that UK consumers may be caught out by confusing rules.

'If you are charged a fee for payment by Mastercard because the card was incorrectly classed as 'business' rather than 'personal', challenge the airline and you should get your money back.

'Companies based in Europe can also continue to legally charge for payments using American Express as they don't have to fall into line with the UK's stronger regulations.

'To avoid the type of charges that we've discovered, think carefully about how you choose to pay for flights with certain airlines and look around to see if you can book the same flight for the same price without paying a surcharge.'

A spokesman for AFKLM added: 'Where Air France and KLM incorrectly applied a credit card surcharge, customers will be refunded.'