The publication of the post-qualifying car weights for this weekend's Spanish Grand Prix in Barcelona has once again thrown up a number of surprises - and cast some doubt upon whether Brawn GP really does still have the fastest car on the grid as the European leg of the 2009 Formula 1 campaign revs into life.

Following the one-hour session around the Circuit de Catalunya - a fraught affair in which Jenson Button flirted with disaster before pulling a scintillating final effort out of the bag to steal pole position away from Red Bull Racing rival Sebastian Vettel right at the close - RBR hinted that Brawn GP may have been carrying less fuel than they were. The statistics prove them right.

The first time in the season that Brawn has felt it necessary to run light in order to preserve their hegemony could be an indication that the ex-Honda F1 outfit's grip at the top of the early pecking order may be beginning to loosen a touch. Button's car weighed in at 5.5kg less than that of Vettel, with Mark Webber in fifth similarly more heavily-laden than the third-placed Brawn of Rubens Barrichello.

Lightest of all inside the top ten is Fernando Alonso, the Spaniard perhaps hoping to put on a good show in front of his adoring partisan supporters but unable to do any better than eighth in his Renault - whilst worryingly for the Brawn-Red Bull quartet is the fact that Felipe Massa, who will begin the race fourth, has more fuel on-board than any of them and a KERS-equipped Ferrari beneath him. Should the Brazilian get the jump when the lights go out, the complexion of the race and strategy choices could alter significantly.

In the intra-team battles, meanwhile, the greatest discrepancy is to be found at Toyota, where seventh-placed Jarno Trulli's performance suddenly appears in an altogether different light. Nine kilograms heavier than team-mate Timo Glock one spot ahead of him in sixth, the Italian - famed for his one-lap heroics - stopped the clocks a scant seven thousandths of a second behind the sister TF109. With more fuel on-board than any of the drivers ahead of him, the Pescara native could just be the race's darkest horse of all...

The full list is as follows (their actual starting position is in brackets at the end):

Fernando Alonso Renault 645.0kg (8th)Jenson Button Brawn GP-Mercedes 646.0kg (1st)Timo Glock Toyota 646.5kg(6th)Rubens Barrichello Brawn GP-Mercedes 649.5kg(3rd)Sebastian Vettel Red Bull Racing-Renault 651.5kg (2nd)Mark Webber Red Bull Racing-Renault 651.5kg (5th)Felipe Massa Ferrari 655.0kg (4th)Jarno Trulli Toyota 655.5kg (7th)Giancarlo Fisichella Force India-Ferrari 656.0kg (20th)Heikki Kovalainen McLaren-Mercedes 657.0kg (18th)Robert Kubica BMW-Sauber 660.0kg (10th)Nico Rosberg Williams-Toyota 668.0kg(9th)S?bastien Bourdais Scuderia Toro Rosso-Ferrari 669.0kg (17th)Kimi Raikkonen Ferrari 673.0kg (16th)Adrian Sutil Force India-Ferrari 675.0kg(19th)Nick Heidfeld BMW-Sauber 676.3kg (13th)Kazuki Nakajima Williams-Toyota 676.6kg (11th)Nelsinho Piquet Renault 677.4kg (12th)S?bastien Buemi Scuderia Toro Rosso-Ferrari 678.0kg (15th)Lewis Hamilton McLaren-Mercedes 683.0kg (14th)