The Advertising Standards Authority (ASA) upheld a complaint made about the ad's claims. It rejected Toyota's argument that the claim was legitimate because it said that the car emitted "up to" one tonne less CO2 than equivalent rivals.
Toyota had given the ASA documentation which outlined the CO2 output of other cars and of the 1.5 litre engine Prius, and another which compared the Prius with previous versions of the car and another Toyota car.
"We noted that none of the cars with 1.5 litre engines featured in the chart emitted 1 tonne more CO2 than the Prius and that less than half of those new cars that had engines of less than 1.8 litres emitted 1 tonne more CO2 than the Prius," said the ASA's ruling.
"We noted that Toyota had qualified the claim by stating 'up to one ton less CO2 per year' and that the Prius emitted significantly less CO2 than some other cars with greater engine capacity, but we did not consider their evidence demonstrated that it emitted 1 tonne less than equivalent vehicles with diesel engines," it said.
The ASA said that the advert breached its TV Advertising Standards Codes on misleading advertising, evidence, implications, environmental claims and comparative advertising.
The ruling came in response to one complaint, and the company was ordered not to broadcast the advert again in the same form.
