Office of Fair Trading chief executive John Fingleton attacked the company for the £5 fee charged on all but one credit card.
The fee has been added to payments with Electron Visa debit cards from 1 January leaving a prepaid MasterCard as the only way to pay without being hit with the charge.
Under the law there must be at least one method on offer in which customers are not clobbered.
Fingleton said: “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.”
He also questioned the automatic addition of insurance to flights by airlines such as Ryanair unless customers opt out.
But Ryanair hit back at the claims, insisting the airline was offering value for money.
Head of communications Stephen McNamara said: “Ryanair is not for the overpaid John Fingletons of this world, but for the everyday Joe Bloggs who opt for guaranteed lowest fares because we give them the opportunity to fly across 26 European countries for free, £5 and £10.
"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.”