George Michael has suffered a series of medical problems since 2011.
The singer, 50, has been out of the spotlight since last year, when he fell out of a car on the M1 motorway and was airlifted to hospital with a head injury.
In 2011 he suffered severe pneumonia which left him temporarily in a coma and meant he needed to undergo a tracheotomy in order to breathe.
He has also battled depression and drug addiction.
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The Islamic centre where the Manchester suicide bomber is thought to have prayed says the attack "has no place in Islam or any religion".
German chancellor Angela Merkel and Indian prime minister Narendra Modi among those to praise "great spirit" of Manchester people.
The bomber's identity has been confirmed by Greater Manchester Police.