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.
Michael's publicist Connie Filippello said in a statement that he had been "diagnosed with pneumonia and he is currently receiving treatment".
Austria's state broadcaster reported that he was being treated in Vienna's AKH hospital.
Michael was due to appear at the Stadthalle in Vienna on Monday but cancelled two hours before the concert was due to start.
He was due to perform in Strasbourg last night and at Cardiff's Motorpoint Arena on Saturday and Sunday but those shows have all been postponed "due to his ongoing illness", a statement on the singer's official website said.
Ticketholders have been told that concerts will be rescheduled and Ms Filippello said new dates "will be provided when available".
It is not the first time that Michael 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.
The concert has now been rescheduled for May 2 at the same venue.
Michael announced his European tour in May - three years after he said he would quit live performance.
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."
Symphonica - The Orchestral Tour kicked off in August at the Prague State Opera House.
During the opening night, he told the audience that the concert would be "a very different evening with George Michael", adding: "I've got to tell you, I'm s******* myself."
Earlier this month, the star played a special gig at the Royal Opera House in London which raised money for Sir Elton John's Aids Foundation.
A star-studded audience including Sir Elton, David Walliams, Kylie Minogue and James Corden were at the show in which the former Wham! singer was accompanied by a symphony orchestra.