George Michael has been taken to hospital with pneumonia, his publicist said on Wednesday night.
Michael was said to be receiving treatment in Vienna where he was due to perform as part of his Symphonica tour on Monday. The show was cancelled two hours before it was expected to start.
The 48-year-old has also been forced to postpone dates in Strasbourg and Cardiff, where he was set to appear at the Motorpoint Arena this weekend.
In a statement, Michael's publicist Connie Filippello said he had been "diagnosed with pneumonia and he is currently receiving treatment". She added new tour dates would be scheduled "when available".
Austria's state broadcaster said the singer was at Vienna's AKH hospital.
Michael announced the 65-show tour of Europe 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."
It is not the first time the singer has cancelled dates in the tour.
He pulled out of appearing at the Royal Albert Hall on October 26 when he suffered a viral infection and high temperature.
Earlier this month, Michael performed for Sir Elton John's Aids Foundation at the Royal Opera House in London.
Stars including Sir Elton, David Walliams and Kylie Minogue attended.
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