The airlines, which have not been named, have been told to bring their practices in line with EU consumer laws within 30 days.

The potentially misleading practices identified by the Commission and EU consumer watchdogs include “creating the incorrect impression that paying an additional fee to finance climate projects with less environmental impact or to support the use of alternative aviation fuels can reduce or fully counterbalance the CO2 emissions”.

They also include the use of the term “sustainable aviation fuels”, without clearly justifying the environmental impact of such fuels, or the use of terms like “green”, “sustainable” or “responsible” in an absolute way.

Other practices highlighted by the Commission include “presenting consumers with a ‘calculator’ for the CO2 emissions of a specific flight, without providing sufficient scientific proof on whether such calculation is reliable and without the information on the elements used for such calculation”.

Věra Jourová, the Commission’s vice-president for values and transparency, said: “If we want responsible consumers, we need to provide them with accurate information.

“More and more travellers care about their environmental footprint and choose products and services with better environmental performance. They deserve accurate and scientific answers, not vague or false claims.

“The Commission is fully committed to empowering consumers in the green transition and fighting greenwashing. We expect airlines, as well as any other industry operator, to make a responsible use of environmental claims.”