The score was authenticated last year
A previously unknown piece of music by Mozart has been discovered at a library in western France.
Ulrich Leisinger, head of research at the International Mozarteum Foundation in Austria, said the single sheet of music was "really important".
"His handwriting is absolutely clearly identifiable," he added. "There's no doubt that this is an original piece handwritten by Mozart."
The sheet was found among the archives by staff at a library in Nantes.
Mr Leisinger said the municipal Mediatheque library contacted his foundation to ask for help authenticating the work.
The score appears to be for a "Credo in D major".
There is a second piece which looked like a "first draft, in parts illegible," said a library official.
It was part of the collection of Pierre-Antoine Laboucheroe, a 19th-century collector who donated his legacy to the city.
The score was catalogued as part of the library's collection, but was later "entirely forgotten" about.
It was rediscovered by the library as it re-catalogued its archives, and authenticated by a researcher from the Mozarteum Foundation last year.
Mr Leisinger said there had been just 10 Mozart finds of such importance over the past 50 years.
If sold, the single sheet would likely fetch around $100,000.
Mr Leisinger said it was the "draft for a piece that Mozart did not work out, for whatever reason".
"It's a melody sketch so what's missing is the harmony and the instrumentation, but you can make sense out of it," he said.
"The tune is complete. It's only one part, and not the whole score with eight or twelve parts.
"One can really get a feeling of what Mozart meant, although we do not know how he would have orchestrated it."
He added: "The fact that an entirely new sheet shows up is extremely rare."