Michael Schumacher's return to Formula 1 in Valencia later this month remains subject to a medical clearance after his spokeswoman Sabine Kehm revealed that concerns linger over the strength of his neck.
The seven-time world champion was injured in a motorcycle testing accident earlier this year, and Kehm admitted that until he had been given the all-clear there was still a possibility that he may not race in the European Grand Prix on August 23.
"It is not sure yet the neck will hold and the comeback can be started," Kehm said on Wednesday.
"It depends on medical investigations. Only after those will it be clear whether he will be able to drive."
Schumacher has already spent a day testing a two-year-old F2007 Ferrari F1 car at Mugello, using GP2 slicks, and claims to have lost 3kg in training as he prepares to stand in for the injured Felipe Massa in Spain.
The 40-year-old continued his build up today by going karting with Ferrari's regular test driver Luca Badoer.
"Karting has a very good training effect, that is why today I just jumped into a testing day of the Tony Kart Team in Lonato," he told his website. "There is nothing better for my preparations at the moment - in terms of steering wheel forces for example Karting is even more difficult than F1.
"And the fact that it has been boiling hot here is simulating the temparatures expected for Valencia. Luca is with me, and together we found again today that karting is just an awful lot of fun."
Kehm added that it was always going to be the case that Schumacher would only race if he was deemed fit enough to do so: "From the very beginning it was made clear Michael would require medical checks. It is normal.
"Michael is fully geared up, and fully on it. He has been for a week now. But he is a human being, and we have to make sure everything is fine."