Red Bull is furious that the test-sheet topping Brawn GP and the Toyota and Williams cars run with a rear diffuser they say is contrary to regulations.

The ruling FIA has declined to place a ban on the uniquely-designed panels despite receiving objections, so Red Bull says it will force the issue to ensure the three teams comply with the standard design used by the other teams.

The diffuser, an underbody section, aids the flow of air and the parts used by the trio are considered by some in pit lane to give them an unfair aerodynamic advantage.

Red Bull adviser Helmut Marko and Renault boss Flavio Briatore are leading the outcry.

''It's illegal,'' Marko said. ''We'll make a protest on Thursday if the component isn't modified to conform to the regulations, because that diffuser guarantees a five-tenths [of a second] advantage per lap. Seven teams are certain it's illegal.''

Briatore said the design should have been declared illegal.

''It looks like there are two sets of regulations: the one that allows some teams to have the diffuser built in a certain way that is forbidden to others because it's considered illegal,'' Briatore said.

''At least three teams don't respect the regulations. It is not what we expected. We want black and white rules for everybody.''

The issue surfaced at the start of pre-season testing and initially involved designs used by Toyota and Williams but intensified when the new Brawn GP cars arrived and began topping time sheets.

The FIA has declined to intervene with president Max Mosley reportedly saying the issue rested with race stewards in Melbourne. Mosley said the matter might have to be settled in the FIA Court of Appeal.

''It's a very clever device and you can make a good case for saying it's legal and a very good case for saying that it's illegal,'' Mosley said.