A handwritten score of one of Ludwig van Beethoven's most revolutionary works has been discovered by a librarian cleaning out a cabinet in a seminary in Pennsylvania after being missing for more than a century.

The 80-page manuscript for a piano version of Grosse Fuge, thought to have been written by Beethoven himself, dates from the final months of his life when he was completely deaf. The work was described by scholars of the German composer yesterday as an "amazing find" and "extremely important".

The lost work came to light in July when Heather Carbo, a librarian at the Palmer Theological Seminary outside Philadelphia, was cleaning out an archival cabinet.

"It was just sitting on the shelf, I was in a state of shock," she told the New York Times. "I'd heard oral history about a Beethoven manuscript, so I recognised what I had found immediately."

The fate of the manuscript has been one of the great musical mysteries since it was auctioned in Berlin in 1890. It is written mainly in brown ink and was described by the New York Times as a furious scattering of notes across the page, with many changes and crossings out.

The work, which went on display yesterday at the evangelical seminary, will be sold at Sotheby's in London on December 1 and is expected to fetch around £1.5m.

Grosse Fuge was composed as the finale for the string quartet in B flat major, Op 130, which Beethoven began in May 1825 and completed in September of that year. It is a notoriously difficult work, and when first performed the audience apparently demanded encores of only two of the movements. "Why not the fugue?" Beethoven demanded. "Cattle! Asses!" he is reputed to have shouted. But despite criticism by contemporaries it is now seen as one of his most important works.

The composer later produced a version for piano, and it is a manuscript of that reworking that has been discovered in Pennsylvania. Stephen Roe of Sotheby's said it was an amazing find: "The manuscript was only known from a brief description in a catalogue in 1890 and it has never been seen or described by Beethoven scholars. Its rediscovery will allow a complete reassessment of this extraordinary music."

Maynard Solomon, a biographer of Beethoven and a world expert on the composer, who has seen a selection of pages from the manuscript, said it was an extremely important find. "It is in beautiful condition and has many interesting compositions and will be the subject of much analytical work because it fills an important gap in the compositional history of one of Beethoven's major works."

Dr Roe said the manuscript was written in brown and black ink, sometimes over pencil, and includes annotations in pencil and red crayon. It shows the extent of Beethoven's reworkings and includes deletions, corrections and deep erasures - occasionally the paper is rubbed right through leaving small holes - smudged alterations and several pages pasted over the original or affixed with sealing wax. The passion that Beethoven endured is also in evidence on the manuscript: the higher and more intense the music becomes the larger the notes.

"What this document gives us is rare insight into the imponderable process of decision-making by which this most complex of quartet movements is made over into a work for piano four hands," Richard Kramer, a musicologist at the Graduate Centre of the City University of New York, told the New York Times.

The manuscript last surfaced at an auction in Berlin in 1890 where it appears to have been purchased by William Howard Doane, a Cincinnati industrialist and hymn writer.