Ryanair has been ordered not to repeat an advertisement that played down the impact of aviation on the environment.
Ryanair says it will defy the ASA and continue to use the figure
In a press campaign the airline claimed the airline industry "accounts for just 2% of carbon dioxide emissions".
The Advertising Standards Agency (ASA) ruled it breached rules on truthfulness by not explaining the figure was based on global rather than UK emissions.
Ryanair claimed the ASA was attempting to suppress an accurate statement, which it would continue to use.
Ryanair's claim that aviation made up 2% of CO2 emissions was based on global carbon dioxide emissions, the ASA said.
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.
But 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 watchdog launched its investigation after receiving 34 complaints from the public.
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".