The seven-time F1 world champion, who retired from the sport for the second time in 2012, was injured in an incident in Meribel, France, on Sunday.
Schumacher was airlifted to a nearby hospital, with director general of the ski resort Christophe Gernignon-Lecomte initially telling radio station RMC that the accident was "not very serious".
However, the 44-year-old German was then moved to the University Hospital of Grenoble, which has revealed in a statement that he is in a "critical" condition, with his head trauma requiring "immediate neurosurgical treatment".
A Scuderia Ferrari and Michael Schumacher fan waits in front of the emergency department of the Centre Hospitalier Universitaire hospital, French Alps. Photo by JEFF PACHOUD/AFP/Getty Images
Schumacher is being examined by personal friend Professor Gerard Saillant, an expert in brain and spinal injuries.
The German was also in the care of Saillant in 1999 after breaking his leg at the British Grand Prix.
Earlier on Sunday, a statement released by Sabine Kehm read: "Michael fell on his head during a private ski trip in the French Alps.
"He was hospitalised and is receiving medical care. We ask for your understanding that we cannot give a running commentary on his state of health.
"He was wearing a helmet and was not alone. No one else was involved in the fall."
Former French F1 pilot Olivier Panis talks to journalists in front of the emergency department of the Centre Hospitalier Universitaire hospital in Grenoble, French Alps. Photo by JEFF PACHOUD/AFP/Getty Images
This is not the first time Schumacher's health has come under threat.
In 2009, Schumacher damaged a vertebra, a rib and the bottom of his skull after he was involved in a motorcycle crash in Spain.
The injuries had caused Schumacher the most serious long-term harm of his career, according to doctor Johannes Peil.
Schumacher, who made his debut in 1991, is still by far the most successful driver in F1 history with 91 race wins from 308 races.
Next on the list is Frenchman Alain Prost (51), while defending world champion Sebastian Vettel has 39 wins to his name.