The masterpiece was found by staff at the city's library, and authenticated as an original work of the Austrian composer last September. The score is on display at Nantes Castle until 22 February, reports the BBC. The score was authenticated by Dr Ulrich Leisinger from the Mozarteum University in Salzburg.
The expert said it was an important discovery. It is composed of two musical pieces with a missing portion at the top. Dr Leisinger said: "We immediately saw that this was Mozart's writing. However, it took more time to realise that this something completely new. It was a great surprise and great joy. The first four lines constitute a whole piece, and that's what is interesting."
"There are three lines of music missing," Dr Leisinger said. "You can see traces of it, but we don't know where the missing part is today," the expert added.