Ryanair has been fined €370,000 by the Netherlands Consumer Authority for violations of Dutch consumer protection law.
In a statement, the NCA said the airline's website offered airfares which did not include all foreseeable and unavoidable charges.
It added that consumers in most cases paid considerably more than the airfares that were initially displayed.
It was also fined for failing to offer consumers the option to review information they entered to correct mistakes before finalising their booking.
The airline was also penalised for failing to provide an email address on its website, and for only offering customer services in English.
In a statement Ryanair said the decision was "littered with errors".
"Ryanair will appeal this unfounded decision and confirms that its website fully complies with all EU and Dutch consumer protection regulations," the airline's spokesman Robin Kiely said.