Shanghai - Jenson Button will be out for his third straight season win at the Chinese Formula One Grand Prix on Sunday now that his fast Brawn GP car has been declared legal.

The governing body FIA said on Wednesday that diffusers used by Brawn, Toyota and Williams meet F1 rules, forcing other teams to play catch-up which could take several weeks.

The diffuser is an aerodynamic feature which channels air out of the back of the chassis. It affects the car's downforce which in turn enhances speed and performance, allegedly making the car half a second faster than others over one lap than one without the diffuser.

"That could be the decision of the year," said German driver Nick Heidfeld from BMW Sauber, one of four teams which had appealed before FIA against the diffuser.

Heidfeld named Button favourite again for the Shanghai race because "he has won the first two races and the diffuser is legal."

Ex-champ Fernando Alonso of the Renault team warned ahead of the ruling that the championship "could be more or less decided" because "the Brawns are going to be nearly unreachable for any other team."

Brawn was naturally happy with the FIA decision which confirmed the Briton Button as championship leader on 15 points from two races ahead of his Brazilian team-mate Rubens Barrichello (10).

"The decision ... 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 world championship," said Ross Brawn, the boss of the team formerly run by Honda.

Button was in full command at the season-opener in Brazil and also won the Malaysian race which could not go the full distance due to torrential rain.

While Brawn, Toyota and Williams are tipped to carry the diffuser advantage at least through the Shanghai and Bahrain (April 26) races, the others have to come up with a similar device as soon as possible, but with no testing allowed between races.

The ruling is especially bad news for the big players Ferrari and McLaren-Mercedes.

Ferrari sit on zero points in the standings in a terrible start to the campaign and McLaren are not much better off with one point from world champion Lewis Hamilton's seventh place.

Ferrari boss Ferrari boss Stefano Domenicali promised that his team will "double our efforts" while Mercedes motorsport chief Norbert Haug called for patience.

"You can't do in nine weeks what others took nine months to achieve," said Haug, still hoping for "another small step forward" in Shanghai.

Hamilton won last year's edition on the 5.451-kilometres Shanghai International Circuit, but also bowed to the realities when he said "it would be very encouraging if we could qualify a little further up the grid and be regularly challenging for points."

However, McLaren must also do something for their reputation which has been badly dented over Hamilton's disqualification from the Australia race for misleading race stewards. The team faces a FIA hearing on April 29 and possibly sanctions over the incident.

Good results would do the team good in Shanghai, where two practice sessions are scheduled for Friday, another practice and qualifying on Saturday and the 56-lap race on Sunday. (dpa)