Boeing UK has become the latest firm to have its knuckles rapped by the Advertising Standards Authority (ASA) over an ad claiming its new 747-8 Intercontinental aircraft would emit less than 75 grams of CO2 per passenger kilometre.

A complaint to the ASA challenged the figure, claiming it was dependent on how many passengers were on board the flight. The watchdog partly upheld the complaint on the grounds that Boeing had undertaken its emission calculations based on a 100 per cent occupancy rate for the plane rather than the 79.7 per cent figure used by the government to calculate airline emissions.

Boeing insisted it had taken a conservative approach to calculating the emissions and argued that there were many unknown factors that could affect emissions per passenger kilometre calculations.

But the ASA concluded that "the claim should have been qualified to make clear that the CO2 figure quoted was based on full occupancy of the aircraft, in order to provide consumers with enough information to understand the claim in context", and ruled that in future this should be made clear when the ad runs in the national press.

The ruling comes a month after Boeing's arch rival Airbus faced similar criticism over the green credentials of its new A380. Reports emerged that the company’s claims that the superjumbo will emit just 75 grams of CO2 per passenger kilometre were also based on full occupancy rates.

Reports in The Telegraph claimed that Airbus's calculations were based on full occupancy rates and the assumption that the aircraft will hold 525 seats. However, the first commercial flight of the aircraft from Singapore Airlines boasted just 471 seats, while Emirates and Quantas are reportedly planning to kit out the plane with 489 and 450 seats respectively, meaning that emissions per passenger will be significantly higher than originally suggested by Airbus.