Large text at the bottom of the ad stated: "Biofuels - A sustainable answer to opec's oil".
Environmental journalist, George Monbiot, challenged the accuracy of this statement.
The watchdog considered government research into the sustainability of production of biofuels.
This stated that further testing should be carried out before the production of biofuels could be claimed to be sustainable.
In addition, the ASA concluded that evidence provided by RFA did not substantiate the claims made in the ad.
It ruled that the ad must no longer appear with the claim "Biofuels - A sustainable answer to opec's oil".