Michael Schumacher has revealed a neck injury is giving him concern ahead of his planned Formula One return later this month.

Since confirming his comeback last Wednesday in place of the injured Felipe Massa, Schumacher has been working intently on his fitness.

The seven-time world champion, who was behind the wheel of a 2007 Ferrari on Friday, has lost weight over the past few days.

However, after a crash during a motorbike test in Spain in February when he damaged his back and neck, the 40-year-old is still experiencing some soreness.

"I'm currently in the midst of my preparations for the upcoming race," said Schumacher, who will be back on track in Valencia on August 23 for the European Grand Prix.

"I have already lost three kilograms, even if it is also important for me to build up muscles.

"All in all, the training is going pretty well, although I have to admit my neck pinches a bit.

"We have to get a grip on that as health has priority. That's the clear arrangement made with Ferrari and, by the way, with my wife (Corinna) too."

After almost three years in retirement, Schumacher is on cloud nine thanks to the overwhelming support he has received over the past few days from his fans.

"I really would like to thank all my fans who are keeping their fingers crossed for my plans and for all the positive feedback," added Schumacher on his personal website.

"It's incredible how much support I have been getting from all over the world. It feels as if a flush of positive energy is coming over me.

"I accepted the challenge and as you all know I love challenges. It seems as if my fans love them too."