Paris - The car with which Jenson Button of the Brawn GP team won the opening two races of season is designed according to Formula One rules, the governing motorsport body FIA said on Wednesday.

The FIA Court of Appeal turned down appeals from Ferrari, Renault, Red Bull and BMW Sauber who had contested the legality of a diffuser used by Brawn GP, Toyota and Williams.

"The FIA International Court of Appeal has decided to deny the appeals," FIA said in a statement after a hearing behind closed doors which took place on Tuesday.

The FIA ruling confirmed that of race stewards at the season-opening Australian Grand Prix on March 29. The FIA Court of Appeal "concluded that the Stewards were correct to find that the cars in question comply with the applicable regulations."

FIA said it would provide full reasons for its ruling "in due course."

The diffuser is an aerodynamic feature which channels air out of the back of the chassis. It affects a Formula One car's downforce which in turn enhances speed and performance.

Wednesday's ruling will prompt the other seven teams to redesign their cars as soon as possible.

However, that will likely not take place before May, which could give Brawn, Toyota and Williams the edge again at Sunday's China GP in Shanghai and the Bahrain race on April 26.

The Briton Button goes into the China race as championship leader on 15 points ahead of team-mate Rubens Barrichello (10) after winning the first two races. Brawn lead the constructors' championship with 25 points from Toyota (16.5 points).

Ross Brawn, the boss of the former Honda team, welcomed the ruling along with Toyota principle Tadashi Yamashina.

"The FIA technical department, the stewards at the Australian and Malaysian Grands Prix and now five judges at the International Court of Appeal have confirmed our belief that our cars have always strictly complied with the 2009 technical regulations," said Brawn.

Yamashina said: "I was confident the Court of Appeal would reach this verdict and I am satisfied with it. It is important to stress we studied the technical regulations in precise detail, consulting the FIA in our process, and never doubted our car complied with them."

"This has been a challenging period for Formula 1 and I am pleased this issue is now in the past and we can focus on an exciting season on the track," he said, as the teams could have lost all their points had the diffuser been deemed illegal.

But BMW motorsport chief Mario Theissen said the decision means that "seven teams will have to invest heavily in carrying out the necessary modifications to their cars."

Ferrari boss Ferrari boss Stefano Domenicali promised that his team will "double our efforts" in an uphill battle.

"Unfortunately this decision forces us to intervene on fundamental areas of the car's design in order to be able to compete on an equal footing with some of the teams...and that will take time and money," he said.

Ferrari have zero points and their rivals McLaren just one point from world champion Lewis Hamilton's seventh place in Malaysia, which makes swift success even more pressing for the two top sides.

Renault's double world champion Fernando Alonso warned ahead if the ruling that if the diffuser design was ruled legal the championship "could be more or less decided" because "the Brawns are going to be nearly unreachable for any other team."