THE HAGUE (AFP) - Forgotten for two decades, a dusty old canvas hidden in a corner of a small Dutch museum has been revealed as a painting by American artist James Whistler.

"Thanks to chemical analysis and an examination of its origins, we have concluded that we have an authentic Whistler," museum curator Jan Rudolph de Lorm told AFP.

The unsigned painting - called Symphony In White. Girl In Muslin Dress - depicts a young girl with black hair dressed in a high-necked, ruffled white frock staring enigmatically straight out of the picture.

The work was displayed in 1956 when the Singer museum in Laren, in the central Netherlands, opened its doors.

But a specialist in the works of the 19th century American-born artist - best known for his painting Arrangement In Grey And Black No. 1 and Whistler's Mother - cast doubts on the authenticity of the canvas and it was relegated to a store room.

"I saw the picture in the depot and I just couldn't get her stare out of my mind," said Rudolph de Lorm.

"A good portrait is like a meeting, and I have met this young woman."

The curator asked for an investigation to be opened and, together with universities in Amsterdam, Antwerp and Glasgow, minutely traced the painting's origins.

Analysis has also revealed traces of the chemical element strontium, matching that found in another Whistler painting exhibited in the central Rijksmuseum in Amsterdam.

Carefully removing traces of restoration work carried out at the start of the 20th century also exposed "very recognisable Whistler brush strokes," Rudolph de Lorm said.

Little is known about the subject of the portrait, though she is thought to be most likely the daughter of an art collector at that time.

Today, the painting is worth about €500,000 (S$800,000).

Born in a New England mill town in 1834, Whistler spent much of his life living in England and became known for his nocturne images of his adopted home of London. He died in the British capital in 1903.