Several budget airlines could be fined more than €1m each for charging passengers for extras that were previously included in the price, such as hand luggage and seat reservations for children to sit next to their parents.

Spain’s Ministry of Consumption has opened disciplinary proceedings against a number of low-cost carriers that have started charging supplements.

While the airlines facing sanctions have not been named, the Ministry previously said the budget carriers being investigated carry close to a third of all passengers.

The Ministry’s investigation was sparked by a complaint from Spanish consumer association Facua that British Airways’ sister airline Vueling was charging for hand luggage, but it has not been confirmed that the Spanish budget carrier is one of those facing sanctions.

The Ministry’s concern is that by removing hand luggage and seat reservations from the ticket price, the budget airlines are able to advertise very low fares, which then means they rank higher in search engines such as Google.

In a statement, it said: “This allows them to have a privileged SEO positioning in search engines and in comparators compared to competitors that do include these services in the price of the ticket that is advertised in the comparator.”

Such practices violate Article 47 of Spain’s Consolidated Text of the General Law for the Defense of Consumers and Users.

Airlines face fines of between €10,001 and €100,000 in the case of serious offences, and between €100,001 and €1,000,000 euros in the case of very serious ones.

However, if the benefits gained from the infringing practices exceed the fines, the sanctions could be increased to up to eight times the illicit benefit obtained.