Barcelona - Early season dominators Jenson Button and Sebastian Vettel are expected to feel the heat from the established teams now that the Formula One season moves to Europe.

Often named the second start of the season, the Spanish Grand Prix on the Circuit de Catalunya in Barcelona on Sunday will see an improved BMW, with McLaren-Mercedes and Ferrari also out to rebound from a poor season start.

"The beginning of the European Formula One season in Barcelona will be a true test for the future balance of power," said Mercedes motorsport chief Norbert Haug.

New rules and the double diffuser used by Button's Brawn GP team and others shook up the hierarchy in the first four season races and prompted the others to play catch-up.

Button, a three time winner from Australia, Malaysia and Bahrain, leads with 31 points from his team-mate Rubens Barrichello (19) and Red Bull's driver Vettel (18), who won the other race in China.

McLaren-Mercedes world champion Lewis Hamilton is stranded on nine points while Ferrari's only season tally is three points from ex-champ Kimi Raikkonen.

Brawn are aware of the challenge and will do their best to keep the momentum going by constantly improving their car.

Team principal Ross Brawn said that level of performance from Bahrain must be sustained "if we are to seriously challenge for the championships this year."

Button said: "We know that we have a real fight on our hands from here to maintain our lead in the championships but I am confident that we are well prepared for the challenges ahead."

Hamilton came fourth two weeks ago in Bahrain but the bad news is that the McLaren does not perform perfectly in the Barcelona track, as indicated in pre-season tests.

"Going back to Europe almost feels like a fresh start. After such a positive performance in Bahrain, we're fully aware that the characteristics of the Barcelona track won't suit our car so well," said the Briton Hamilton.

"It will be interesting to see where we sit in the order."

Ferrari say they have also done their homework for the 66-lap race on the 4.655-kilometres course where Raikkonen triumphed last year and where a good result in Saturday's qualifying is important as overtaking is difficult.

"The Ferrari F60 will be fitted with a new aero package that was always scheduled to debut at the Spanish Grand Prix. In addition, the package also includes some extra changes, originally scheduled for the Turkish and British events, but now brought forward," Ferrari said on its website.

BMW have also made plenty of changes in an effort to salvage a season they entered with serious title ambitions but has so far only brought four points from Nick Heidfeld.

"I hope that we will perform rather better against our rivals in Barcelona, but that is difficult to predict as all the teams will be bringing upgrade packages of varying degrees to the first race of the European season," said Heidfeld.