The Dutch airline KLM has misled customers with vague environmental claims and painted “an overly rosy picture” of its sustainable aviation fuel, a court has found.

In a greenwashing case brought by the campaign group Fossielvrij, the district court of Amsterdam ruled on Wednesday that KLM had broken the law with misleading advertising in 15 of the 19 environmental statements it assessed. They include claims that the airline is moving towards a “more sustainable” future and statements on its website about the benefits of offsetting a flight.

“Today’s judgment is a landmark victory in the fight against greenwashing,” said Hiske Arts, a campaigner at Fossielvrij. “The court could not have been clearer: companies are not allowed to claim they are tackling dangerous climate change when in reality they are fuelling the crisis.”

Several of the claims KLM made about its environmental ambitions were declared misleading because they were too vague. In relation to a billboard at Amsterdam’s Schiphol airport showing a child on a swing, the court ruled that KLM’s statement “join us in creating a more sustainable future” did not explain how flying with it related to any environmental benefit. The impression was reinforced by the background of sky, mountain and water, it said.

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The court also took aim at the airline’s presentation of its policies on sustainable aviation fuels, a vital but fledgling solution to the sector’s emissions, and tree-planting, which is sold as a way to offset the emissions from a flight. “These measures only marginally reduce the negative environmental aspects and give the wrong impression that flying with KLM is sustainable,” it ruled.

Four statements that campaigners had criticised were deemed fair to use. The court also said the airline did not have to rectify the incorrect statements or warn customers that aviation is currently not sustainable. But it said that when KLM informed its customers about ambitions to reduce emissions, it must do so “honestly and concretely”.

Campaigners said the court’s verdict was an important victory against greenwashing. but warned it was not enough. “Tackling greenwashing is currently a cat-and-mouse game,” said Rosanne Rootert, a campaigner at Reclame Fossielvrij. “If one misleading campaign is stopped, 10 new ones emerge. And you can only respond once the damage has already been done: people have already seen the commercials.”

A similar point was made by KLM’s lawyer Branda Katan, a professor of corporate litigation at Leiden Law School, before the verdict.

Katan, who is scheduled to give an inaugural lecture at Leiden University on Friday titled: “Sustainability through liability? Don’t get your hopes up”, saidcivil lawsuits were slow and the results only said something about a specific case. “With climate litigation, you grab attention – media attention – and, at least that is the hope, you instil fear in other companies. So interest groups are using it to try to bring about behavioural change.”

Though the air transport sector accounts for only a small fraction of greenhouse gas emissions today, its contribution to climate breakdown is on course to soar as demand rises. The International Energy Agency found last year that growth in aviation activity has historically outpaced improvements in efficiency.

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Rootert said the “next logical step” was for the EU to restrict fossil fuel adverts as it had done with tobacco. “A complete ban of fossil advertising, such as for air travel, is the only way to truly eliminate greenwashing by these companies.”

A KLM spokesperson, Marjan Rozemeijer, said the airline had not used the 19 communications addressed in the court case for some time. “We are pleased that the court has ruled that we can continue to communicate with our customers and partners about our approach to making aviation more sustainable. We are continuously learning how best to include them in this.”

She added: “It is good that the court gives us more clarity about what is possible and how we can continue to communicate transparently and honestly about our approach and activities.”