The Office of Fair Trading has accused Ryanair of being “puerile and childish” over its payment policy.

OFT chief executive John Fingleton attacked the budget carrier’s stealth charges when customers use all but one type of credit card to pay online.

He told The Independent that the firm used a legal loophole to justify charging the extra fee.

But Ryanair has defended their payment policy model, saying it allows passengers “to avoid costs”.

The law says that if an airline offers at least one free payment method, it is allowed to advertise cheap fares that do not include extra credit card charges.

Fingleton said the airline had chosen a rare payment method to get around the rules.

Currently the airline only offers a free booking service to customers who pay for tickets with a MasterCard prepaid card.

He added: “Ryanair has this funny game where they have found some very low frequency payment mechanism and say: ‘Well because you can pay with that’”.

“It’s almost like taunting consumers and pointing out: ‘Oh well, we know this is completely outside the spirit of the law, but we think it’s within the narrow letter of the law’.”

He added: “On some level it’s quite puerile - it’s almost childish.”

Ryanair Head of Communications Stephen McNamara said: “What the OFT must realise is that passengers prefer Ryanair’s model as it allows them to avoid costs, such as baggage charges, which are still included in the high fares of high cost, fuel surcharging, strike-threatened airlines such as BA.”

He added: “Perhaps Mr Fingleton’s comments are designed to cover over the OFT’s failure to take any action against BA’s unfair fuel surcharges, the BAA’s monopoly pricing or the continuing mis-selling by screenscraper websites across the UK and Europe, who routinely add hidden mark-ups to Ryanair’s low fares.”