Former Formula 1 chief doctor Gary Hartstein has warned that fans must prepare themselves "for the worst" news as attempts to awaken Michael Schumacher continue.

The legendary German racer has been in a state of induced coma since suffering a head injury while skiing in the French Alps in December 2013.

Although earlier in March Schumacher's manager Sabine Kehm released a statement, saying "small, encouraging signs" have been seen at times, Dr Hartstein has admitted that the longer it takes, the less likely it is that Schumacher will ever recover.

"As time goes on it becomes less and less likely that Michael will emerge to any significant extent," he said on his personal blog, the Eurosport reports.

"I always knew Michael was adored. I'm still staggered by the depth and persistence of his fans' love for him," Dr Hartstein wrote on his blog.

According to him, the lack of media updates on Schumacher's condition has given the fans a chance "to move on, to process what's happening, and to start to... detach."

Earlier reports claimed that Schumacher had lost up to 25% of his body weight due to muscle atrophy, a condition common for patients in a deep coma.