Greek singer Demis Roussos, who sold more than 60 million albums worldwide, has died aged 68, the Hygeia Hospital in Athens has confirmed to the BBC.
He was best known for his solo hits in the 1970s and 80s, including Forever and Ever, Goodbye and Quand je t'aime.
He was also a member of progressive rock group Aphrodite's Child.
Roussos was renowned for his off-screen role in Mike Leigh's 1977 TV play Abigail's Party, having provided the party's soundtrack.
He had been in the private hospital with an undisclosed illness for some time, and died surrounded by his family
His Aphrodite's Child bandmate Vangelis paid tribute in a statement that begins: "Demis my friend.
"I have just arrived in London and I've been told that you decide to take the long voyage, I'm shocked because I can't believe that this happened so soon.
"Nature gave you this magic voice of yours which made millions of people around the world very happy."
He added: "As for me, I keep those special memories that we share together those early days and I wish you to be happy wherever you are."
He signed off with the words: "Goodbye my friend goodbye. Love Vangelis."
Greek singer Nana Mouskouri paid tribute on French radio RTL: "He had a superb voice, he travelled in the world ... he loved what he was doing.
"He was an artist, a friend. I hope he is in a better world."
The singer was born Artemios Ventouris Roussos in Alexandria, Egypt, in 1946, to a Greek father and Egyptian mother of Italian origin.
He was raised there until his parents moved to Greece in the early 60s after losing their possessions during the Suez Crisis.
Roussos began his music career at 17, when he joined the a band called The Idols, where he met Vangelis.
Aphrodite's Child produced three albums including It's Five O'Clock and 666, and enjoyed huge success in Europe in the late 1960s, especially France.
Roussos went on to enjoy a successful solo career, topping the charts in several countries with Forever And Ever in 1973, before doing the same in the UK in 1976.
Memorably he was referred to in Mike Leigh's Abigail's Party, by the character played by Alison Steadman, who plays his record in an attempt to impress her guests - commenting that he "doesn't sound" fat.
Other solo hits include My Friend the Wind, My Reason, Someday Somewhere and Happy To Be On An Island In The Sun.
Roussos' fondness for kaftans saw him dubbed "the Kaftan King" and he often wore them for his performances on shows such as Top of the Pops.
He was also famous for his vocal adaptation of the score from the 1981 film Chariots of Fire, which had been composed by Vangelis.
In 1978, he decided to keep a lower profile and moved to Malibu Beach in the US - where he shed much of the weight that had seen him routinely mocked by comedians like Freddie Starr.
Famously, he was caught up in a plane hijacking when flight TWA 847 from Athens to Rome was hijacked by members of Hezbollah and Islamic Jihad in 1985.
He and his third wife were held at gunpoint for five days before they were released. Some of his fellow passengers endured 17 days in captivity.
The experience changed his life and afterwards he decided the best way he could help others and promote understanding in the world was by returning to music.
He released his album The Story of Demis Roussos not long after.
The star is survived by his mother, Olga (94), children Emily and Cyril, long-term partner Dominique, brother Costas and ex-wives Pamela and Monique.
A funeral will be held on Friday, 30 January in Athens, his manager Denis Vaughan told the BBC.
"We will miss the amazing Demis, whose singing brought sunshine to the world," Vaughan added.
"He was a legend. He played hard, he worked hard. The world is a less fun place without Demis".