Fair trading officials have clashed with Ryanair over the reason the Irish airline gave for introducing extra charges for its passengers.
Ryanair said it was bringing in a 2% processing fee on all new credit card bookings made from Friday.
The no-frills carrier, which is also scrapping exemptions from its £6 website administration fee, said it was introducing the £2 fee "in order to comply with the UK Office of Fair Trading's (OFT) recent ruling on credit card fees".
But the OFT said: "We have not required any airline to introduce new payment charges, increase their credit card charges or scrap any discounts they wish to offer."
The OFT added: "We took action to make sure that debit card charges are included in the headline price and credit card charges are transparent and not sprung on shoppers towards the end of the booking process."
Those using Ryanair's "cash passport" scheme in Ireland, Germany and Spain will be able to avoid administration fees until February 1, February 15 and March 21 2013 respectively.
Ryanair said it was continuing to "deliver the lowest fares and a no-fuel surcharges guarantee to all our passengers", and that customers could avoid credit card fees by paying with a debit card.