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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.
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."