"George Michael has severe community acquired pneumonia and is being treated as an inpatient. His condition has stabilized and he is responding to treatment," the singer's doctors said in a statement issued to Britain's Daily Mail newspaper Thursday.
"From the current point of view, the time until recovery cannot be estimated, but he will not be able to perform the rest of the tour. Besides medical treatment, complete rest and peace and quiet are mandatory."
The 48-year-old singer has been performing with a symphony orchestra on his Symphonica tour since August.
Michael was due to appear in Vienna Monday but fell ill shortly before he was to take the stage. He was transported to Vienna's AKH hospital where he is being treated.