World motorsport's governing body the FIA has ruled that F1 championship leader Jenson Button's car is legal.
Button has won both races this season and there were complaints that his Brawn GP team and rivals Toyota and Williams used an illegal diffuser.
But following eight hours of strongly worded evidence the International Court of Appeal ruled that the cars "comply with the applicable regulations".
All three teams are free to race in the Chinese Grand Prix in Shanghai.
"The decision comes as no great surprise," said BBC Radio 5 Live Formula 1 commentator David Croft.
"Already this season two sets of stewards, an FIA president and an FIA race director have thought that the diffuser design of Brawn, Williams and Toyota was OK.
we had every confidence that the design of our car would be confirmed as legal
"It's a sensible decision for Formula 1. The FIA's court of appeal were highly unlikely, on technical grounds, to go against the decision of the stewards. Seven teams are now playing catch up and have to do something about it very, very quickly."
Brawn GP team boss Ross Brawn said in a statement following the verdict: "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 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 GP currently lead the constructors' world championship with 25 points, with Toyota in second place on 16 points.
A statement from the Japanese team said: "Our team studied the wording of the new 2009 regulations in precise detail to ensure we interpreted them correctly.
"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.
WHAT IS A DIFFUSER?
It is the rear part of the floor of the car between the rear wheels and under the rear wing
It is crucial to the aerodynamics, and small changes can have a big impact on downforce - and therefore grip and speed
"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."
Williams chief executive Adam Parr admitted his relief at the outcome and predicted the rival teams would install rear diffusers "within days".
BMW motorsport director Mario Theissen said the team would accept the decision but didn't consider it fully legitimate.
In a statement Ferrari team principal Stefano Domenicali added: "We are waiting to hear the reasons the ICA rejected the appeal.
"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 from a point of view of the technical regulations, and that will take time and money.
"We will now double our efforts to get the team back to the highest level of competitivity."
The judges in Paris heard evidence from both sides, with Ferrari's legal representative, Nigel Tozzi QC, describing Brawn GP team boss Ross Brawn as "a person of supreme arrogance".
Brawn defended himself robustly and insisted his team's diffuser was simply "an innovative approach of an existing idea".
And Brawn's criticism of Ferrari consultant Rory Bryne and Red Bull technical guru Adrian Newey saw sparks fly in the courtroom, with the Englishman refusing to retract his statements.
FIA technical director Charlie Whiting was also grilled, with accusations his organisation are "getting it wrong, and not understanding the point".
The row broke out when Ferrari, Renault and Red Bull protested against the legality of the split-level diffusers on the eve of the season-opening grand prix in Australia, but race stewards in Melbourne rejected their claims.
BMW Sauber then had a similar protest rejected at the Malaysian Grand Prix while McLaren have recently added their weight to the official protest.
Speaking from outside FIA headquarters in Paris on Tuesday, BBC sports news reporter Joe Wilson said: "Rear diffusers this season were supposed to get smaller, but Brawn and a couple of other teams have ended up with bigger ones, exploiting a little loophole in the laws."
The protesting teams said the split-level design contravenes a rule that states the diffuser - an aerodynamic body part which aids performance - must have an upper edge that runs in a horizontal straight line..
They also believed the design is negating the main aim of this season's new rule changes, which is to make overtaking easier
The split-level diffusers generate more downforce at the rear of the car, resulting in a clear performance advantage of around 0.5 seconds per lap.
With the FIA's ruling, the seven teams who are running without the split-level diffusers are expected to try to incorporate the design into their cars as soon as possible.
But setting about such a radical redesign with the season under way will not be easy.
Many of the teams running with regular diffusers argue the cost of making the changes is too great, especially during a climate of cost-cutting within the sport in the face of the global economic crisis.
"I've heard several of them complaining about the cost," said Brawn before the verdict.
"But there are lots of things teams copy from each other and the cost doesn't get debated.
"I think some teams will be able to do it very quickly, but for other teams it will be more difficult because of their suspension configuration or other elements of the car."
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