Formula One legend Michael Schumacher is to turn 45 on Friday while lying comatose in a French hospital after suffering critical head injuries in a ski accident.

Doctors and family members were tight-lipped Thursday about the condition of the German racing great, who has been hospitalised since Sunday, when he slammed his head against a rock while skiing in the French Alps.

There have been conflicting statements about the speed the seven-time world racing champion was going when the tragedy befell him in the Meribel ski resort, where he has a property. The rock impact split the helmet Schumacher was wearing in two, according to a source close to the investigation.

The Ferrari F1 team where Schumacher spent many years announced it would on Friday hold a "silent gathering" in front of the Grenoble hospital to mark the birthday of the retired sportsman, who is being kept in an induced coma.

But Schumacher's fan club in his childhood town of Kerpen said any celebration of the birthday would be in bad taste.

Jean Todt, former head of the team, visited Schumacher's bedside on Thursday. Schumacher's wife Corinna, their two teenage children, and his father and brother were also there.

The hospital and Schumacher's manager had briefed the press daily since the accident. But they did not do so on Thursday, instead promising to communicate only if there was something new to report.

Schumacher has had two operations to remove bleeding and pressure on his brain.

His media representative, Sabine Kehm, said Wednesday Schumacher was stable though still critical.

Hospital under intense media pressure

The hospital has been faced with intense media pressure from the worldwide interest in its famous patient. A vacant lot nearby has been turned into an impromptu parking lot for numerous television satellite vans.

Kehm earlier in the week said some people had tried to sneak into Schumacher's room, one dressed as a priest.

The three medical professors treating him -- two neurosurgeons and the head of the anesthetic and intensive care department -- have also been thrown into the spotlight.

They have appeared at the two press conferences to explain the latest developments. While they have refused to speculate on how Schumacher's condition may evolve, they agree that his age and fitness could help with recovery.

Schumacher's accident has prompted an outpouring of sympathy from fans shocked to see their idol hovering so close to death after having cheated it so many times on the racing track.

Yet questions have emerged over exactly how the accident happened in a small, seemingly innocuous off-piste section of Meribel located between two ski slopes -- one classed easy and the other intermediate.

Prosecutors have opened a probe into the accident, as is common practice in France in such cases, and are exploring the theory that Schumacher was skiing at great speed when he fell. That factor could go to establishing liability or negligence, depending on circumstances.

The prosecutors are also looking at whether the limits of the pistes next to the area where accident happened were correctly marked, and whether the safety releases on Schumacher's skis operated properly. The German newspaper Bild reported the skis were rented and one of their safety releases did not open at the moment of the accident.

Prosecutors were not expected to make any statement on the investigation until early next week.

Kehm told reporters this week that the former racer was not skiing fast when he fell.

"He seems to have hit a rock as he took a turn. It was a chain of unfortunate circumstances," she said.

She added that Schumacher was with his 14-year-old son Mick at the time of the accident, as well as a small group of friends.

"He was not going quickly, because it seems he helped a friend who had fallen down," she said.

Schumacher dominated Formula One from his debut in 1991, winning more world titles and races than any other driver.

He first retired aged 37 but was unable to resist the lure of the track. In 2010, he came out of retirement but was unable to recover his previous performance and quit for good in 2012.

As an F1 racer, Schumacher was known for his daring overtaking manoeuvres, his at-times almost reckless abandon in the pursuit of victory and his mastery of tricky conditions presented by rain.