The 53-year-old singer died from heart failure at his home in Goring-on-Thames, Oxfordshire, South East England, on Christmas Day (25.12.16) and the Beatles legend had led the tributes to his late friend.
He posted a picture of them together on Twitter, taken after they worked on the song 'Heal the Pain', and wrote: ''George Michael's sweet soul music will live on even after his sudden death.
''Having worked with him on a number of occasions his great talent always shone through and his self-deprecating sense of humour made the experience even more pleasurable.''
Robbie Williams tweeted: ''Oh God no ...I love you George ...Rest In Peace x Choose life my friends xx (sic).''
George's Wham bandmate Andrew Ridgeley wrote on Twitter: ''Heartbroken at the loss of my beloved friend Yog. Me, his loved ones, his friends, the world of music, the world at large. 4ever loved. A xx (sic).''
And Shirlie Holliman, who was a singer for Wham and met her husband through George, wrote: ''Beyond heartbroken. Words cannot express how sad we all are, only last week I saw him laughing and happy. My heart is broken to lose someone so special.''
Her husband Martin Kemp added: ''My whole family and I are devastated at the loss of our beautiful friend Yog! We will miss him so much! We are all heartbroken!''
And their son Roman also paid tribute, saying: ''The man who toured the world with my mum; her best friend.
''The man who introduced my parents; who forced my mum to call my dad
''They man that took me and Harley around the world; just to see us smile.
''The man we all love. We love you Yog.''
Geri Halliwell shared an Instagram picture of her and George and wrote: ''So so sad, R.I.P George, a dear friend, the kindest, most generous & talented person. My thoughts are with his family (sic).''
Bryan Adams was another star who shared his grief on Twitter, writing: ''RIP George Michael. I can't believe it. Such an incredible singer and a lovely human being, far too young to leave us #georgemichael (sic) and Ellen DeGeneres added: ''I just heard about my friend @GeorgeMichael's death. He was such a brilliant talent. I'm so sad (sic).''
However, Sarah Michelle Gellar was left embarrassed when she mistakenly paid tribute to Boy George rather than George Michael in a Twitter post.
She wrote: ''Do you really want to hurt me? I guess you do 2016 #ripboygeorge I was truly one of your biggest fans (sic).''
However, after fans pointed out her mistake, she deleted the message and wrote: ''Just as sad when you get the correct information- #ripGeorgeMichael thank you to everyone who corrected me- it's still so sad
''And for the record yes I completely know the difference between Boy George and George Michael- I heard incorrectly. My intentions were good
''This is usually why I don't comment on public matters, but it all seemed so sad on Christmas. Lesson learned (sic).''
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