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The piece for organ by composer Johann Sebastian Bach (1685-1750) is a variation on an old German choral melody entitled "Wo Gott der Herr nicht bei uns haelt," the university said. The "fantasy" variation takes about five to seven minutes to play, officials said.

Two musicologists came across the composition among the 19th-century papers of Wilhelm Rust (1822-1892), the editor of Bach's first complete works. They had been sorting through the papers which the University of Halle-Wittenberg library acquired at an auction a few week ago.

Baroque composer Bach is considered one of the greatest composers of organ music in history.

Dated around 1705

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The musicologists, Michael Pacholke and Stephan Blaut, who are doing research for a book on Bach's complete works, dated the piece to between 1705 and 1710, based on its stylistic features.

"Bach split up the choral melody -- which was a well-known hymn -- into single lines and embellished them for the organ, devising some motifs of his own," Blaut told DPA news agency. Editor Rust copied it by hand in 1877.

It remains a mystery why Rust -- who was an organist at Bach's old church, St. Thomas in Leipzig -- chose to leave the piece out of his 26-volume "Complete Works of Bach," which he had begun editing in 1858.

The "fantasy" variation was originally thought to be lost, with only the first five bars known. It had been recorded in Bach's catalogue raisonne as a "work of questionable authenticity."