An unknown waltz by Chopin, written nearly 200 years ago, has been discovered in the vault of the Morgan Library and Museum in New York.

The score, on a card bearing Frederic Chopin's hand-written name, was found by a curator in the spring, the New York Times reported Sunday.

"I thought, 'What's going on here? What could this be?' I didn't recognize the music," curator Robinson McClellan told the paper.

He was at first unsure that the piece was actually Chopin's after photographing the score and playing it on a keyboard at home.

He conferred with an academic at the University of Pennsylvania who is an expert on Chopin, before the Morgan concluded the find was genuine after testing the ink and paper.

The penmanship was also found to match Chopin's, including the reproduction of a stylized bass clef symbol as well as doodling characteristic of the composer.

"We have total confidence in our conclusion," McClellan said.

The museum believes that the music is from between 1830 and 1835, when Chopin was in his early 20s.

The tune features a stark opening and was described by pianist Lang Lang as containing "dramatic darkness turning into a positive thing."

Chopin, who wrote mostly piano solos, died aged 39 in France in 1849.