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ROME, Italy — A missing Gustav Klimt masterpiece that vanished from an Italian gallery, only for it to reappear within its walls 23 years later, has been confirmed by art experts as the original.

"It's with no small emotion that I can tell you the work is authentic," Ornella Chicca, a prosecutor in the northern city of Piacenza, told reporters on Friday at a news conference.

The "Portrait of a Lady" painting, valued at $66 million, vanished from the city's Ricci Oddi Modern Art Gallery, during renovation work in February 1997.

Believing it stolen, Italian police launched a wide ranging investigation and came up empty handed.

Then in December, almost 23-years after it vanished, it was discovered hidden inside the gallery's walls.

While clearing ivy, a gardener who has not been named, noticed a metal panel. He opened it and found a bag inside a space within the walls.

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Museum employee Dario Gallinari told NBC News that he was coming back from his lunch break when the gardener told him they found something and handed him a trash bag.

"I immediately saw a corner of a painting sticking out of the bag and recognized it straight away," he said, adding that he had seen pictures of the painting for his entire life, but never the original.

"I even remember the day it was stolen," he said. "I was 9 and I was at school when I found out and we have lived with the mystery of its disappearance ever since.

"So you can imagine how emotional it was to have it suddenly in my hands. I immediately run into the museum and showed it to a colleague of mine. He also couldn’t believe his eyes.”

Painted between 1916 and 1917, the painting of the woman sensually glancing over her shoulder against a dreamy green background is a later work by the Austrian art nouveau master.

Its disappearance had been one of the art world's biggest mysteries.

There had been widespread optimism in Italian art circles that the gardener's discovery would turn out to be the missing Klimt.

Italy's Piacenza Sera newspaper quoted gallery officials as saying the back of the canvas bore stamps that were put on when the painting was on loan.

Since its discovery, the work had been kept in a vault of a local branch of Italy's central bank.

Claudio Lavanga reported from Rome and Henry Austin from London.