Formula One championship leader Jenson Button's title hopes received a significant boost yesterday when a Paris appeal court ruled the Briton's controversial Brawn GP car was legal.
The FIA's court of appeal also cleared Toyota and Williams, whose cars use similar 'double-decker' or 'split-level' rear diffusers and which rivals had claimed broke the spirit of the rules.
Button won the first two races of the season in Australia and Malaysia after overcoming protests from struggling champions Ferrari, Renault, Red Bull and BMW-Sauber. The four teams had then appealed against the stewards' decisions.
The FIA said in a statement that the court, which met on Tuesday, had decided "to deny the appeals submitted".
"Based on the arguments heard and evidence before it, the Court has concluded that the stewards were correct to find that the cars in question comply with the applicable regulations," the statement added.
Button and team-mate Rubens Barrichello will now be favourites to complete a hat-trick of Brawn victories in Sunday's Chinese Grand Prix in Shanghai.
"We respect the right of our competitors to query any design or concept used on our cars through the channels available to them," said Brawn principal Ross Brawn.
"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."
Brawn have taken over the Honda team operation, with the Japanese manufacturer pumping hundreds of millions of dollars and more than a year of effort into their new car before deciding to pull out in December.
Had the verdict gone against the "diffuser three", Button could have been stripped of his wins with the championship thrown into turmoil.
Instead, rivals will now be racing to come up with their own versions of the diffuser, which ensures a smooth flow of air under the car to create more downforce, at a time when testing is banned.
Renault have already said they have a prototype that could be used in China.
"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," said Ferrari principal Stefano Domenicali, whose team have yet to score a point in their worst start since 1992.
"That will take time and money."
Toyota, currently second in the constructors' championship behind Brawn after finishing on the podium in both races, said in a statement they had played by the rules.
"We studied the technical regulations in precise detail, consulting the FIA in our process, and never doubted our car complied with them," said team principal Tadashi Yamashina.