George Michael's voice could be permanently affected by his battle with community acquired pneumonia.
The 48-year-old singer was hospitalised in Austria last week and while doctors say his condition is improving, his vocal cords could be left damaged by the treatment he is receiving.
Ear, nose and throat surgeon Gerrit Wohlt said: "In the worst case scenario, being on a ventilator could harm his vocal cords. He will need months of rest.
"It depends how long he spends on the ventilator, but in the worst case his illness could seriously affect his singing capacity.
"The pressure is very close to his vocal cords and it could affect them."
Meanwhile, the 'Outside' singer's family - 75-year-old father Kyriacos Panayiotou, who is known as Jack Panos and sisters Melanie, 49, and 53-year-old Yioda - have flown out to Vienna to keep a bedside vigil, joining his boyfriend Fadi Fawaz, who has been at the hospital's intensive care unit since the star was taken ill.
A source told the Daily Mail newspaper: "His whole family are with him. They are very close-knit. George is just such a lucky man in that sense. He's so adored."
George's former boyfriend Kenny Goss - who he was in a relationship with for 13 years until 2009 - is also believed to be flying out to Austria.
The source added: "Kenny of course is very concerned. He has been in touch every day. They will always be the best of friends."
George has cancelled the remaining 14 dates of his 'Symphonica Tour' after falling ill.