Online prices for air fares must include taxes, charges, fees and other add-ons from the beginning of the booking process, Europe's highest court ruled on Thursday.
The European Court of Justice warned airlines against the practice of adding on fees - such as for using credit cards - at the final stage of paying for flights.
The decision comes at the end of an almost decade-long battle between budget carrier Ryanair and the Italian Competition and Market Authority (AGCM).
"An air carrier, such as Ryanair, is obliged to indicate in its online offers, from the first time that the price is shown (i.e. in the initial offer), the air fare and, separately, the taxes, charges, surcharges and fees that are unavoidable and foreseeable," the ECJ's ruling reads.
"By contrast, it is required to indicate the optional price supplements in a clear, transparent and unambiguous way only at the start of the booking process."
Ryanair was criticised in 2011 by the AGCM for not having indicated on its online prices "unavoidable and foreseeable" charges including "the amount of VAT on domestic flights, the online check-in fees, and the fees charged when paying by a credit card other than that approved by Ryanair".
"The AGCM considered that those price elements were unavoidable and foreseeable and that the consumer therefore had to be informed of them from the first time the price was indicated, that is to say before a booking process was commenced," the ruling noted.
The AGCM fined Ryanair for unfair commercial practice, leading to several appeals, first before the Italian administrative courts, then before Italy's Council of State.
The Italian Council of State asked the European Court of Justice about the case, leading to Thursday's ruling.
Euronews has reached out to `Ryanair for comment.