PARIS, April 14 (Reuters) - Formula One's international court of appeal meets on Tuesday to decide on the legality of the Brawn GP car that took Jenson Button to victory in the first two races of the season.

Champions Ferrari, Red Bull, Renault and BMW-Sauber are united in appealing against stewards' decisions in Australia and Malaysia that the rear diffusers used by Brawn, Toyota and Williams were legal.

A verdict is not due until Wednesday afternoon, by which time the teams will be already in Shanghai preparing for Sunday's third round of the season in China.

McLaren, whose Mercedes engines also power the Brawn, will attend the hearing in Paris as well but their representatives are not expected to speak despite sharing the view of the protesting four teams.

Ferrari will be represented by their former title-winning designer Rory Byrne, who was a close colleague of Brawn principal Ross Brawn at the Italian team, and chief designer Nikolas Tombazis.

The controversy centres on the rear diffuser, which channels the air cleanly under and out of the rear of car to create maximum downforce.

Rivals claim the "double-decker" ones used by Brawn, Toyota and Williams break the spirit of the rules. Brawn, who have been comfortably quickest in both races so far, and Toyota are currently first and second in the standings.

"This decision will have an enormous impact on the championship," Ferrari's 2007 world champion Kimi Raikkonen, whose team have yet to score a point this season, said on his website (www.kimiraikkonen.com).