Ryanair has seen an advertisement promoting cheap flights banned by the Advertising Standards Agency after the watchdog found the price claims breached its code.
The advert, which ran nationally in the press in February, claimed customers could fly to Malmo in Sweden from £14.99 one way.
However, the promotion did not make clear that the £14.99 was subject to customers paying with a pre-pay MasterCard or Ryanair's Cash Passport Card, and the ASA said it was concerned that customers who didn't have either of those cards could miss out on the offer.
In its defence, Ryanair said the advertised fare was available to all consumers who chose to avoid optional fees and charges and said the footnote on the ad made that clear.
The airline also said the initial cost of the Cash Passport was £6, but that this cost was immediately redeemable against the cost of the first flight purchased using the card.
But customers who didn't have a Cash Passport at the point of purchase could not buy the tickets for the advertised price; instead they would have to apply for the card and wait for it to be delivered; and the delay could result in consumers being unable to pay the advertised price.
"We also understood that in order to use the Cash Passport, consumers were required to load the card with a minimum of £150 and that additional charges applied to its use," the ASA said in its judgment.
"We considered such details to be material information which the consumer needed in order to make an informed decision about the advertised price.
"Because the surcharge was not avoidable at the point of purchase unless a consumer was already in possession of the Cash Passport, and because in order to obtain the advertised price consumers were required to pay using the Cash Passport, which was subject to significant conditions the ad did not make clear we concluded the ad breached the Code."
Ryanair introduced its Cash Passport card in 2011.
It immediately faced criticism from consumer lobby group Which? for charging excessive fees, including a fee for withdrawing cash from the card over the counter at a bank or from an ATM, a 50p charge for all transactions, other than Ryanair bookings, from April 2012 and a rolling fee of £2.50 if a card is not used for six months.
And the airline came in for more criticism this summer after a report in the Telegraph found British customers were being charged up to 25% more than other Europeans due to the British pound strengthening against the euro.