George Michael is in hospital suffering from pneumonia.
The 48-year-old singer has cancelled a string of appearances on his Symphonica tour while he is being treated.
Austria's state broadcaster reported that he was being treated in Vienna's AKH hospital.
George was due to appear at the Stadthalle in Vienna on Monday but cancelled two hours before the concert was due to start.
It is not the first time that George has been forced to disappoint his fans due to ill health.
He pulled out of appearing at the Royal Albert Hall on October 26 when he suffered a viral infection and high temperature.
George announced his European tour in May - three years after he said he would quit live performances.
At the time, the singer - who spent a month in prison for drug driving last year - told a press conference at the Royal Opera House in London:
"I felt very re-energised after my recent troubles, I felt it was really what I want to do as an artist."