A painting found hidden inside an Italian gallery's walls has been confirmed as Austrian painter Gustav Klimt's "Portrait of a Lady," which went missing more than two decades ago.

The piece, which was recovered by chance in December in the northern Italian city of Piacenza, was stolen 23 years ago. It was found on December 10 in the courtyard of the Ricci Oddi Modern Art Gallery — the same place where it was stolen in 1997 — while the gallery was undergoing renovation.

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A gardener reported finding the artwork inside a bag last month while clearing ivy at the gallery.

"It is with no small emotion that I can tell you the work is authentic," Piacenza Prosecutor Ornella Chicca told reporters at a news conference.

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The portrait, which depicts a young woman looking over her shoulder against a green background, was on display on an easel surrounded by two police officers during the official announcement made by the prosecutor.

"I can finally announce to all people from Piacenza, to all scholars, to all friends that the work "Portrait of a Lady" produced by Gustav Klimt and owned by the #GaleriaRicciOddi is authentic!," tweeted Laura Bonfanti, the vice president of the gallery.

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The recovered painting is one of a series of portraits of women Klimt painted towards the end of his life, between 1916 and 1918. Its disappearance had been one of the art world's biggest mysteries — but questions remain over who took the artwork and if it ever left the gallery's property.