Ferrari's legal representative Nigel Tozzi told the court in Paris on Tuesday there was no doubt the teams have exploited a loophole in the regulations to use an illegal device.
"Anyone with a command of English will tell you it is a hole, so do not let someone attempting to be clever with words defeat the express purpose of the rules," Tozzi was quoted as saying by the Press Association.
"The appeal is not because we have not made the most of an opportunity, but because Brawn, Toyota and Williams have not acted within the regulations."
Renault's representative Andrew Ford said the sport's governing body had already concluded the design was illegal, and that was the reason why the French team decided not to use it.
"It is not that Renault missed the boat, as Brawn have pointed out, it is because the FIA said it was illegal. It was at that point the diffuser was abandoned," said Ford.
