Singer George Michael has died at his home at the age of 53.
His manager, Michael Lippman, said he had died of heart failure.
Former Wham! bandmate Andrew Ridgeley said he was "heartbroken at the loss of my beloved friend".
Writing on Twitter and referring to Michael by his family nickname of "Yog", he added: "Me, his loved ones, his friends, the world of music, the world at large. 4ever loved. A xx".
On Instagram, Sir Elton John posted a photograph of himself with Michael, writing: "I am in deep shock. I have lost a beloved friend - the kindest, most generous soul and a brilliant artist. My heart goes out to his family and all of his fans."
South Central Ambulance Service were called to Michael's property at 13:42 GMT. Thames Valley Police also attended and Michael was confirmed dead at the scene.
They added: "At this stage the death is being treated as unexplained but not suspicious. A post-mortem will be undertaken in due course. "
Home village mourns George Michael
A small heart wreath and a rose are among the tributes left outside the front door of Michael's home, a detached property by the River Thames.
Court battleMichael, who was born Georgios Kyriacos Panayiotou in north London, sold more than 100 million albums throughout a career spanning almost four decades.
He first found fame with schoolfriend Ridgeley in duo Wham! - reaching number one in the UK singles charts on four occasions. One of the band's most enduring songs Last Christmas, is currently number 16 in the UK singles chart.
The song was originally released in 1984 and is the biggest selling single not to reach number one.
It was kept off the chart's top spot by Band Aid's Do They Know It's Christmas, which actually featured Michael.
Solo albums followed, including the multi-million selling Faith in 1987.
The follow-up Listen Without Prejudice Vol. 1 outsold Faith in the UK but led to Michael losing a court case with record label Sony over his frustration at how the album has been marketed.
His talents as a singer, songwriter and music producer made George Michael one of the world's biggest-selling artists.
Blessed with good looks and a fine singing voice, his stage presence made him a favourite on the live concert circuit as he matured from teen idol to long term stardom.
But there were times when his battle with drugs and encounters with the police made lurid headlines that threatened to eclipse his musical talents.
Read more in George Michael's obituary
As a solo artist, Michael scored a further seven number one singles in the UK with songs including Careless Whisper and Fastlove, collaborated with the likes of Aretha Franklin and Elton John, and won three Brit Awards and two Grammys.
The Grammy organisers said Michael's "extraordinary talent had a profound impact on countless entertainers worldwide, and his creative contributions will live on forever".
Troubled artistMichael later began facing headlines for reasons other than his music.
After years of refusing to be drawn on speculation about his sexuality, Michael disclosed he was gay in 1998 after being arrested in a public toilet in Beverly Hills, California, for engaging in a lewd act.
The music video for the single that followed, Outside, featured a men's bathroom transformed into a disco and policemen kissing.
In October 2006 he pleaded guilty to driving while unfit through drugs, and in 2008 was cautioned for possession of class A drugs, including crack cocaine.
In September 2010, Michael received an eight-week prison sentence following an incident in which he crashed his Range Rover into a shop in north London. He admitted driving under the influence of drugs and possessing cannabis.
Pop stars from Michael's heyday were joined by contemporary names in paying tribute on Twitter.
ABC's Martin Fry said: "Absolutely devastated to hear of the loss of @GeorgeMichael Truly brilliant talent #sad #sad #sad."