The Spanish Grand Prix has in recent years symbolised the beginning of the European leg of the Formula 1 campaign, but in 2009 it is likely to take on an even greater significance still, with most predicting that the weekend will reveal the true pecking order following the opening four flyaway races.

In Australia, Malaysia, China and Bahrain, three teams dominated proceedings at the front of the field - namely Brawn GP, Red Bull Racing and Toyota. Of those, only the first two have triumphed - Brawn courtesy of world championship leader Jenson Button in Melbourne, Sepang and Sakhir, and Red Bull storming to not only its breakthrough victory in the top flight after 74 starts but indeed its first one-two with Sebastian Vettel and Mark Webber in the downpour of Shanghai.

Similar conditions are unlikely in Barcelona, but Brawn and RBR can nonetheless expect to once more be up at the business end of the grid, with few question marks over the capabilities of early-season team leaders Button and Vettel. By contrast, both Webber and Rubens Barrichello in the second of the Mercedes-powered BGP001s need to produce a strong performance on Spanish soil if they are not to risk finding themselves rapidly relegated into rear-gunner roles to their respective team-mates' championship tussle.

Toyota, for their part, will be determined to avenge for a hugely frustrating Bahrain Grand Prix last time out, when after locking out the front row for the first time in its history, the big-budget Japanese manufacturer slipped away on race day as a result of a misguided strategy choice, with Jarno Trulli doing extremely well to hang onto the final rostrum position but Timo Glock managing to take the chequered flag no higher than seventh, having led the initial eleven laps. Toyota has the pace to win, the drivers to win and the money to win, but has yet to put all the pieces of the puzzle into place at the same time - could Barcelona finally put that unenviable record to bed?

Behind F1's current top trio, Williams look in danger of sliding down the starting grid as better-financed rivals make inroads into their advantage, with any benefit afforded by the Grove-based former multiple world champions' 'double-decker' diffuser now likely to be negated by upgrades from other teams. Williams has not made the most of its Toyota-powered FW31 package over the first four grands prix of 2009, and it knows it - the question now is to whether Sir Frank's boys can maintain development pace to match their bigger and wealthier competitors.

Of those teams looking to steal a march this weekend, McLaren-Mercedes, Ferrari and Renault look to be the best-placed, with all three recognising that in the absence of a substantial step forward around the Circuit de Catalunya, their title chances may be shot. Double F1 World Champion Fernando Alonso in particular is desperate to shine in front of his partisan supporters, after lining up on the front row of the grid on home turf for the past three years in succession.

BMW-Sauber, too, has promised considerable gains under the Spanish sun, with the Bavarian outfit one of the undisputed disappointments thus far in 2009 - and Nick Heidfeld and Robert Kubica having come home respectively last and last-but-one in Sakhir. Improvements for the Munich and Hinwil-based concern need to be both significant - and swift.

It will be fascinating to see whether the sport's traditional 'grandees' have done enough to redress the balance in Barcelona, whilst the remaining two contenders - Scuderia Toro Rosso and Force India - arguably have an even greater mountain to climb, with, rare exceptions apart, both having been glued firmly to the back of the pack for much of the season to-date.

S?bastien Buemi has shown well for the Red Bull 'junior' outfit and team-mate and namesake S?bastien Bourdais belatedly seemed to kick his challenge into gear in Bahrain, but still the small Faenza-based squad is struggling to make its STR4 perform to anything like the same standard as its big brother, the Red Bull Racing RB5. Force India, finally, have pointed to progress at each race so far in 2009 - the only trouble for the Silverstone-based minnows, is that everybody else seems to be progressing just as fast...'s Tip for the Top: Rubens Barrichello's One to Watch: Fernando Alonso