The company has been banned from using two of its adverts which promote a money-saving fuel because they have been ruled to be misleading.
A direct mailing advert, sent in March, said Shell scientists had developed a regular fuel to help consumers save money.
It said: “Their latest fuels – Shell FuelSave Unleaded and Diesel – are designed to help you save fuel and money. These advanced fuels each have a special formula enriched with Shell Efficiency Improver combined with a special detergent package – designed to improve your fuel economy from the very first fill.”
The advert featured a man dressed up in a laboratory coat holding a full one-litre measuring glass with the promise that customers could save up to one litre per tank at no extra cost.
The second advert, broadcast on the radio in April, told consumers that Shell could help them save fuel and money.
Three complainants challenged these claims because they believed that the adverts exaggerated the benefits available.
Shell argued that its tests showed that both the unleaded and diesel fuels achieved a 2% saving more than 10% of the time.
The Advertising Standards Authority upheld the claims and said the adverts must not appear or be broadcast again in their current form.
Its judgment said: “The ASA noted that the ads stated that fuel savings ‘may vary according to vehicle’, but we considered that the claim in the ads that consumers could save up to one litre per tank at no extra cost implied that the saving would be applicable to all or most vehicles.
“We acknowledged that, where advertisers made ‘up to’ savings claims, the Committee of Advertising Practice and the ASA generally expected that they should be able to demonstrate that 10% of consumers would be able to attain the maximum saving claimed.
“We therefore considered that the claim should be substantiated by evidence showing that at least 10% of vehicles in the UK would save one litre of fuel in every 50-litre fill-up.
“We noted that Shell had provided substantiation which showed that over 10% of the car models they had tested for both the unleaded and diesel versions of FuelSave would, on average, achieve fuel savings of at least 2%.
“However, we considered that data relating to just four models of car for each fuel did not constitute adequate substantiation that 10% of all motorists could achieve one-litre fuel savings in every 50-litre fill-up.”