The national press ad for the airline claimed that an easyJet plane emitted 22 per cent less CO2 than another plane on the same route.
But the Advertising Standards Agency today ruled that it did not make it clear that the figure related to emissions per passenger.
It said: "We concluded that, because the basis for the claim had not been fully explained, the ad misleadingly implied that easyJet planes were more environmentally efficient than the aircraft used by traditional airlines."
It said the figure was primarily based on the fact that easyJet planes could carry more passengers than traditional airlines.
The ASA told easyJet to ensure that the basis for comparisons was explained in future ads.
The airline was also cleared of misleading over a claim in the same ad that flying could be 50 per cent cleaner within ten years.
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