The FIA International Court of Appeal has declared the double-decker diffuser designs used by Brawn GP, Toyota and Williams as legal.
Following overnight deliberation by the judges after a court hearing in Paris on Tuesday, the ICA has rejected the appeals lodged by Ferrari, Red Bull Racing, Renault. BMW Sauber and McLaren had also entered the appeal as affected parties.
A statement issued by the FIA on Wednesday morning said: "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.
"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 ICA's decision is a blow to those teams that did not pursue the design concept when they created their 2009 cars - as it is widely accepted that the double-decker diffusers have brought a performance advantage.
Ferrari's Kimi Raikkonen suggested last week that the diffuser decision would be vital for the outcome of the world title - with his team likely to have to wait for several races before being able to fit a suitable one to their car.
"The FIA's Court of Appeal will decide about the diffuser and this decision will have an enormous impact on the championship," Raikkonen said.
"We're missing grip and downforce. You just need to analyze the performance in the three sectors at Sepang to understand that we're losing a lot compared to the best cars. You could see it especially in the middle sector where downforce is really crucial.
The row over the diffusers has also led to intense confrontation between the rival factions – with Brawn GP team principal Ross Brawn being on the receiving end of attacks from Renault and Ferrari about his use of the diffuser concept.
However, he has stood firm in his belief that the design was legal – and confirmed recently that he offered rivals the chance to close off the regulations to prevent teams exploiting the diffuser designs, but they rejected the opportunity.
"In March 2008 that was offered," said Brawn, when asked by AUTOSPORT magazine about the matter.
"If I'm frank I didn't say 'look we are going to do this diffuser if you don't accept this rule' because I'm not going to tell people what we're doing, but I explained that I felt that we should have a different set of rules to simplify what needs to be done.
"I offered them and they were rejected, so my conscience is very clear. And those rules that I put on the table would have stopped a lot of things. It would have stopped the diffuser, it would have stopped all those barge-boards around the front, and it would have cleaned the cars up.
"Because it was clear that when we started to work on the regulations that there were things that you could do, and we needed to perhaps clean them up, but nobody was interested. They are interested now."