Ryanair has been ordered not to repeat an advertisement that played down the impact of aviation on the environment.
In a press advertising campaign, the airline claimed the airline industry 'accounts for just 2% of carbon dioxide emissions'.
The UK's Advertising Standards Agency (ASA) - after receiving 34 complaints from the public - has ruled that it breached rules on truthfulness by not explaining that the figure was based on global - rather than UK - emissions.
The UK government figures for domestic and international flights leaving the UK, put the figure at 5.5%. "Because Ryanair had failed to make the basis of the 2% figure quoted in the ad sufficiently clear, it was likely to mislead" - the ASA said.
However, Ryanair argued its 2% figure came from a UN report on climate change and that it used a global figure because the issue was a global one.
The European Environment Agency's executive director, Professor Jacqueline McGlade, welcomed the ruling, saying that Ryanair had sought to 'trivialise' the impact of aviation on the environment. She described the airline's approach to CO2 emissions as "disingenuous and intellectually dishonest".
It is the second time this year that the carrier has got into trouble for misleading environmental claims. In January it conceded - following a BBC investigation - that a claim it had cut its CO2 emissions by half in recent years was 'a mistake'.
The airline has reacted to the latest ASA ruling in its usual 'no-holds barred' style by slamming the ASA for - "attempting to cover up Greedy Gordon Brown's travel tax scam, which was introduced in February and will raise over UK£1bn under false pretences from airline passengers this year".
Ryanair, which claims to be 'Europe's greenest and cleanest airline' added that - "as part of this cover-up, the ASA quango is attempting to suppress factually accurate statements made by Ryanair". It has said that it will continue to use these "factually accurate statements" in its advertising for as long as "this travel tax scam continues".