Porsche has not had any diesels in its range since February.
The Tonight programme explores the conundrum facing car buyers as the car industry shifts towards hybrid and electric vehicles.
The environmental group claimed the results undermined industry claims that new diesels were the "cleanest in history."
Two of Britain's leading supermarkets have dropped the price of diesel by 2p to less than 98p per litre.
Asda, Tesco and Sainsbury's will sell the fuel for less than the symbolic level from today.