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 ruled on Wednesday.
Brawn drivers Barrichello and Button have had a successful - and fair - start to the F1 season
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 the decision 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 down force which in turn enhances speed and performance.
Wednesday's ruling will prompt other 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 Shanghai GP and the Bahrain race on April 26.
Brawn welcomes decision
British driver Button goes into the China race as championship leader on 15 points after winning the first two races. The Brawn team leads the constructors' championship with 25 points ahead of Toyota (16.5 points).
Brawn hailed the verdict confirming the legality of the diffuser.
Button's Brawn car has been deemed within the rules
"We are pleased with the decision reached by the International Court of Appeal today,” the team's statement said. "We respect the right of our competitors to query any design or concept used on our cars through the channels available to them."
"The FIA Technical Department, the Stewards at the Australian and Malaysian Grand 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," the statement said.
Brawn added that the decisison "brings this matter to its conclusion and we look forward to continuing on the track the challenge of what has been a very exciting start to the 2009 FIA Formula One World Championship."
Williams, Toyota also involved
The controversial diffuser was also fitted to the cars of the Williams and Toyota teams, and Toyota said they had always been confident of the outcome going in their favor.
"As we have maintained throughout, our team studied the wording of the new 2009 regulations in precise detail to ensure we interpreted them correctly," a statement from the Japanese team, which is based in Cologne, Germany, said.
It went on: "We also made full use of the consultation procedure with the FIA which was a helpful process to ensure our interpretation of the technical regulations was correct.
Toyota glad to see the back of F1's "challenging time"
"Therefore we had every confidence that the design of our car would be confirmed as legal, firstly by race stewards in Australia and Malaysia and subsequently by the Court of Appeal."
Toyota's Timo Glock (far right) has also benefited
Toyota team boss Tadashi Yamashina welcomed the FIA ruling. "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," he said in a team statement.
"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 Renault's double world champion Fernando Alonso warned that if the diffuser design is ruled legal the championship "could be more or less decided" because "the Brawns are going to be nearly unreachable for any other team."
While Brawn have a runaway lead from Australia and Malaysia races, Ferrari find themselves with zero points and McLaren have just one point from world champion Lewis Hamilton's seventh place in Malaysia.