Williams wants established drivers

Rosberg said in Spain that while he remains extremely happy at Williams, he is considering his options as he wants to be in a race-winning car by 2010.

Williams already has Formula 3 Euro Series champion Nico Hulkenberg on its books as reserve driver, but Head said that the German was not necessarily first on line for a race seat if one became vacant.

"He's clearly very talented and we'll obviously keep an eye on how he's getting on," he said. "But in a way Williams needs to get itself back to a position where it employs two strong, established drivers rather than new people.

"As one sees with Sebastian Vettel, and as one saw with Lewis Hamilton in 2007, there are some people that seem to be able to make the transfer and take to it like a duck to water. It's quite difficult to look at somebody in GP2 and see if that's going to be the case."

Head added that Hulkenberg was generally performing well in testing, but had blotted his copybook with some errors when entrusted with new car shakedowns.

"I think he had the wrath of Sam Michael's tongue a couple of times," Head admitted. "Both last year and this year he's driven our new car first on the first day, and in both cases I think after a very short amount of running he ended up in the gravel trap when all one is actually trying to do is sort out systems, gather data and things like that.

"So that hasn't been particularly clever. Equally, recently when he's run that hasn't been the case."