Audi have won six of the last seven races at Le Mans

Audi made history with the first-ever diesel-powered Le Mans victory at the famous French circuit last month.

But questions have been raised over the equivalency formula used to keep different engine types equal.

The Automobile Club de l'Ouest claimed Audi had bettered their closest rivals by 3.2 seconds a lap and said it will "take steps to reduce its performance".

Peugeot plan to join Audi in racing a diesel at Le Mans next year.

But one of the key advantages of the technology - improved fuel efficiency - will be curbed.

The Automobile Club added: "The fuel tank capacity will have to be adjusted in such a way that the cars receive the same quantity of energy at each refuelling stop.

"This will mean a smaller tank for those cars which are equipped with a diesel engine."

Audi drivers Frank Biela, Marco Werner and Emanuele Pirro won Le Mans last month by four laps from their nearest petrol-engined rivals.