Red Bull pose the only real threat to Jenson Button's world championship charge, according to team boss Ross Brawn.

Brawn dismissed his former Ferrari team and defending champion Lewis Hamilton's struggling McLaren outfit after Sunday's Spanish Grand Prix, saying only Sebastian Vettel and Mark Webber could now reel in Button.

No driver has failed to win the F1 title after such a strong start to the season as Button's and Brawn said the belief was there that the Briton could carry on his winning streak, which he stretched to four from five races in Barcelona.

When asked if Red Bull were the sole threat in the drivers' and constructors' standings, Brawn (below) said: “It looks that way, doesn't it? It looks like they and us are the main two teams that understood the opportunities with the new regulations and they've got a very strong team.

“They look to me at the moment that they are the main challenge to us. Other teams will come and go during the season but in terms of the championship and long term, Red Bull look very strong, the only real threat to us.”

Ferrari showed much improved pace at the Circuit de Catalunya and were unlucky to pick up just three points for Felipe Massa's sixth place after the Brazilian struggled with a lack of fuel in the dying laps of the race.

And Brawn praised his former employers for “the huge step from where they were at the beginning of the season”.

But he warned they still had a lot to do. “You don't know if this is a one-off and merely a circuit that suited them better,” he said. “They've still got work to do to start thinking about winning races.”

The only other driver to have won this season, bar Button, is Vettel, taking victory at a rain-affected Chinese Grand Prix.

For the second successive race, however, the German was held up by Ferrari and was unable to show his true potential.

“Things could have been very different for them in the championship standings had Ferrari not held them up,” said Brawn. “They're a big threat and I don't think we've seen their full pace.”

However, Brawn insisted there was still more to come from his team. They are planning minor upgrades for both Monaco a week on Sunday and Britain on 21 June but have a more major overhaul lined up for the German Grand Prix on
12 July. And Brawn is confident it will keep them ahead of the chasing pack for the remainder of the season.

“You never completely know what's going to happen,” he added. “We've got a group of people, they are working hard. They've all got a target for the German Grand Prix and the past history is they make it. That's when the major upgrade will come.”

The biggest problem facing Brawn seems to be the potential fall-out between Button and team-mate Rubens Barrichello.

Barrichello, who quit Ferrari after playing second fiddle to Michael Schumacher under Brawn, has threatened to walk out on the team if he believes they are favouring Button over him.

The Brazilian had looked favourite to win in Barcelona after an electric start but he was outdone by Button, who was changed to a two-stop strategy during the race while Barrichello remained on a three-stopper, leading to suggestions the team had done so to aid Button's championship cause.

And Barrichello warned: “If I get the slightest sniff of the fact that they have favoured Jenson, I will hang up my helmet tomorrow. But I know Ross wouldn't do that. He asked me to drive for him and he knows I want to race fairly with Jenson.”

Brawn, in turn, laughed off suggestions of favouritism within the team.

The 54-year-old insisted: “There is no priority being given and I'd love to see Rubens win a race and see his crew win a race because it would be really great for the team.

“You saw with Rubens' pass at the first corner that we have no team orders. He flew past Jenson and we were perfectly happy about that.”