Nick Heidfeld has admitted that the BMW Sauber team has work still to do before it is capable of challenging the likes of Ferrari and Renault for the Formula One title.

Having focused on supplying engines to the Williams team from 2000 to 2005, BMW increased its Formula One involvement ahead of 2006 with the purchase of the Sauber outfit. Partnering Heidfeld with Jacques Villeneuve, and then Robert Kubica when the Pole replaced Villeneuve towards the end of the season, the team enjoyed a strong debut campaign, only failing to score points on six events and ending the season in fifth place in the constructors' title.

Despite a consistent campaign with the F1.06 that saw him finish in the points in over half the races and claim a podium finish in Hungary, Heidfeld said it was important for the team to keep its feet on the ground going into 2007, when the German will be partnered by promising newcomer Kubica for the full season.

“We assume that we will keep on improving but it will get harder and harder,” he admitted. “It's the goal of BMW to win races but we are well aware that it won't be possible to secure the world championship crown within one year. I would be happy should we succeed in moving up from fifth to fourth position in the constructors' championship.

“[In F1] you currently have to cope with a lot of extremely competitive opponents. Other than in the past, there aren't just three that have got what it needs to win races. The small teams of the past don't exist any longer and we've got some major companies in F1 today.”

However, the former Prost, Sauber, Jordan and Wiliams man insisted that his current team are the most competitive outfit he has been part of during his career to date.

“Most definitely it's the most promising initial position I've been in so far,” he said. “Never before have I raced for such a competitive team. I'm truly optimistic regarding the future.”