The King Crimson and Bucks Fizz lyricist Peter Sinfield has died aged 80.
He also provided the words for Greg Lake's perennial festive hit I Believe In Father Christmas, which first charted in 1975, and for Celine Dion's global smash Think Twice.
Sinfield, originally from London, had lived in Aldeburgh on the Suffolk coast for decades.
King Crimson's website said he had been "suffering from declining health for several years" and died on Thursday.
Sinfield was part of the first line-up of King Crimson, whose 1969 debut In The Court Of The Crimson King is widely regarded as one of the defining progressive rock albums.
Its opening song, 21st Century Schizoid Man, was later sampled by rapper Kanye West on Power.
After Greg Lake left Crimson to form Emerson, Lake & Palmer, Sinfield contributed lyrics to albums including Brain Salad Surgery.
He also produced the first Roxy Music album and hit single Virginia Plain in 1972, and wrote the lyrics for the Bucks Fizz number one Land Of Make Believe in the early 1980s, as well as lyric for their albums.
In the 1990s, he wrote the lyrics for Celine Dion's global hit Think Twice.
He also recorded a solo album called Still in 1974.
King Crimson's management said it believed Sinfield had contracted sepsis and died in hospital.
ELP's drummer Carl Palmer paid tribute to a "great poet and lyricist" on his website.
"Peter will be sadly missed," he said
"A great person to be with and very funny. We were with him at his home about 18 months ago and we talked and talked all afternoon."
Lake, who died in 2016, was also in the original line-up of King Crimson and the pair worked together on Lake's surprise solo Christmas hit.
"I had no expectations of it becoming a hit record at all," Sinfield said in 2014.
Speaking decades after writing the song, the pair differed over what it was about.
Lake said: "The magical Christmas we had both known as children had over the years somehow deteriorated into one huge marketing exercise and we decided it would be a good idea to write a song that resisted this decline in the hope the magic could somehow be restored."
Sinfield said: "There wasn't a sudden revelation of finding out Father Christmas didn't exist, but it happened in 1951 or so when I was seven or eight.
"My parents were divorced and it was tied up with a loss of innocence - finding out that I didn't have a normal family.
"The people surrounding me were all my mother's friends.
"Our German house-keeper kept up the spirit of Christmas and then suddenly it was taken away from me at the age of eight when I was sent off to boarding school."
Although in was kept off the top spot in 1975 by Queen's Bohemian Rhapsody, Sinfield said radio station plays every December meant it kept him in royalties that he estimated were about £20,000 a year.