"For several days stolen documents and data are being offered for sale. The offeror claims them to be the medical file of Michael Schumacher," the German's spokeswoman Sabine Kehm said in a statement.
"We cannot judge if these documents are authentic. However, the documents are clearly stolen. The theft has been reported. The authorities are involved," she added.
Kehm warned against either the purchase or the publication of the data, stressing: "The contents of any medical files are totally private and confidential and must not made available to the public."
She vowed to sue "against any publication of the content or reference to the medical file."
Schumacher was transferred from a French hospital to a facility in Switzerland earlier this month after emerging from a coma.
He had been treated since December 29 in the French Alpine city of Grenoble after he slammed his head against a rock while skiing with his son and friends.
A police source later told AFP that Grenoble hospital had last week filed a lawsuit regarding the theft of the data.
The 45-year-old was transferred to a hospital in the Swiss city of Lausanne where he will be undergoing further treatment.
The racing star underwent two operations to remove life-threatening blood clots after the freak accident that shocked the world, before being plunged into a medically induced coma.
Since then, Kehm had said he was showing short moments of consciousness, but few other details have filtered out and it is still unclear as to what the future holds for the man who cheated death countless times on the track.