Halle, Germany  - A lost musical composition by Johann Sebastian Bach (1685-1750) has been discovered in a treasure trove of manuscripts in Germany, the University of Halle said Tuesday.

The piece, which would take 5 to 7 minutes to play, was a "fantasy" or variation on an old German choral melody titled, Wo Gott der Herr nicht bei uns haelt, the university said.

Two musicologists discovered it in the 19th century papers of the editor of the first complete works of Bach, Wilhelm Rust (1822-1892). The Halle university library purchased his papers several weeks ago.

Baroque composer Bach is one of the greatest authors of organ music ever.

The fantasy had been thought lost, with only the first five bars known.

The musicologists, Michael Pacholke and Stephan Blaut, are working on a new complete Bach and said the piece's stylistic features enabled them to date it to the period
1705 to 1710.

"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," said Blaut. Rust copied it by hand in 1877.

It was not clear why Rust, who was organist at Bach's old church, St Thomas in Leipzig, from 1880, left the piece out of his 26-volume Complete Works of Bach, which he had begun editing in 1858.