The FIA has declared that the controversial diffuser designs used by Brawn GP, Toyota and Williams are legal – rejecting an appeal from rival teams.
The devices were originally protested in Melbourne, where the stewards ruled that they were within the regulations.
Ferrari, BMW Sauber, Red Bull and Renault remained unconvinced and appealed the decision, but after a hearing in Paris yesterday, their arguments have been rejected.
"The FIA International Court of Appeal has decided to deny the appeals submitted against decisions numbered 16 to 24 taken by the panel of the stewards on 26 March at the 2009 Grand Prix of Australia and counting towards the 2009 FIA Formula One World Championship," said an FIA statement.
“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 decision now puts the protesting teams under pressure to follow their rivals' lead and develop their own versions of the taller 'double decker' diffusers, which are widely thought to have been key to Brawn's dominant start to 2009 and Toyota and Williams's improved form.
The appeal court's decision is a massive boost for Jenson Button and Brawn GP's title challenge.
If the verdict had gone against them, they would not only have lost a major element of their current advantage, but had to hurriedly redesign an integral part of their car – as would Toyota and Williams.
Instead it is their rivals that must now urgently address their own designs to take advantage of the same grey areas that the three teams exploited.
While some could have new diffusers ready by the Spanish Grand Prix next month, others – notably Red Bull and Ferrari – are believed to need to make fundamental changes in order to accommodate revised diffusers.
Red Bull Racing boss Christian Horner has admitted his team will have to comprehensively repackage the RB5 and redesign its gearbox.
Today's verdict comes as little surprise to most in the Formula 1 paddock, especially as FIA president Max Mosley had said during pre-season media briefings that he personally regarded the three teams' designs as 'clever' rather than illegal.
But under F1 rules, there was no opportunity to formally challenge the teams involved until scrutineering in Melbourne, and an appeal was always likely whatever the result.
The protesting teams claimed that at best the designs were against the spirit of the revised 2009 aerodynamic regulations, which were intended to cut downforce and clear up rear end airflow to improve overtaking.
Some have suggested that this is a dangerous avenue of development to begin, as it raises the possible of too much grip coming from the underbody, as had been the case under the 'ground effect' rules of the early 1980s.
There were also concerns that costs would rise as other teams hurled resources at developing their own versions of Brawn, Williams and Toyota's diffusers.
But the three teams were always adamant that they had acted within the rules and had simply out-thought their rivals.
What is the diffuser?
The rear diffuser is an upward-sweeping tray on the underside of the car, located beneath the engine and gearbox behind the rear wheel axle line.
While the front and rear wings (aerofoils) work the air that passes over the car's body, the diffuser harnesses the air that travels underneath the car.
The diffuser's shape causes air to be channelled through it and creates a suction effect that pushes the car into the ground.
Working in conjunction with the undertray, the diffuser generates a large proportion of the car's rear downforce.
Why the controversy?
The 2009 regulations restrict the maximum height of the diffuser's central section to 175mm.
But by exploiting regulations that allow extra bodywork within a 150mm zone in the centre of the car, Brawn GP, Toyota and Williams have cleverly shaped their cars' rear crash structure so that it effectively extends the diffuser's central section.
Taken together, the result is a central section that exceeds the 175mm height limit that applies to the diffuser alone.
A taller diffuser means more rear downforce, a commodity that is in short supply this year due to the higher and narrower rear wings.
The three teams who have taken this route are adamant that they have observed the letter of the rules, and FIA president Max Mosley conceded that they have been "clever" in exploiting a grey area.
But rival teams have argued that their interpretation flouts the spirit of the regulations, with Renault boss Flavio Briatore claiming they had simply drawn up their own rule book.
The back story:
Brawn's 'conscience clear' on diffusers
BMW joins diffuser appeal
Diffuser appeal hearing set for April 14
RBR: Diffuser row has big implications
Briatore slams Brawn over diffusers
Stewards throw out diffuser protests
Protests lodged against three teams
Mosley expects trouble over diffusers