If the airline has not respected your rights as defined in Regulation (EC) No. 261/2004, which establishes common rules for compensation and assistance to passengers in the event of denied boarding, flight cancellations, or long delays, you should submit a written complaint to the airline in an informal format.

If, after sending the complaint, the airline does not respond or rejects your claim, you have the right to seek assistance from the regulatory body in the country where the incident occurred. A list of these regulatory bodies is available on the European Commission’s website: https://transport.ec.europa.eu/transport-themes/passenger-rights/national-enforcement-bodies-neb_en