Missing painting found inside Italian gallery wall confirmed as a genuine work of Gustav Klimt

Art experts have confirmed a painting discovered hidden inside an Italian art gallery's walls last month is Gustav Klimt's "Portrait of a Lady", which was stolen from the gallery almost 23 years ago.

The authentication of the painting announced on Friday solved one of the art world's enduring mysteries - where did the missing work end up?

But it also leaves several questions unanswered, including who had taken it or whether it ever left the museum's property.

"It's with no small emotion that I can tell you the work is authentic," Piacenza Prosecutor Ornella Chicca told reporters, while two police officers stood on either side of an easel bearing the recovered painting.

Italian art restorer Anna Selleri points at a slide showing a detail of a painting which was found last December. Credit: AP

"Portrait of a Lady" depicts a young woman sensually glancing over her shoulder against a dreamy moss green background.

Klimt finished the painting in 1917, the year before he died.

A gardener at the Ricci Oddi Modern Art Gallery in the northern city of Piacenza who was clearing away ivy noticed a small panel door on a wall outside and opened it.

Inside the space, he found a plastic bag containing a painting that appeared to be the missing masterpiece.

The Ricci Oddi gallery acquired it in 1925 and reported it missing in February 1997.

Art experts have confirmed that a painting discovered hidden inside an Italian art gallery's walls is Gustav Klimt's 'Portrait of a Lady'. Credit: AP

Since the gardener's discovery on December 10, the canvas had been kept in a vault of a local branch of Italy's central bank while experts used infrared radiation and other non-invasive techniques to determine if it was the original “Portrait of a Lady.”

Experts said the painting was in remarkably good condition.

One of the few signs of damage was a scratch near the edge of the canvas that may have resulted "from a clumsy effort to remove the portrait from its frame," said Anna Selleri, an art restorer from the National Gallery in Bologna.

Now experts say this enigmatic work is undoubtedly worth even more.

Art critic Estelle Lovatt says the oil on canvass is worth even more now because of its story. Credit: ITV News

Art critic Estelle Lovatt told ITV News that "finding this painting now will effect it's value greatly because you're not only buying a painting, you're not only buying an oil on canvass but you're buying a full story."

Another celebrity Klimt piece was a second portrait of the woman, "Portrait of Adele Bloch-Bauer II."

Oprah Winfrey eventually purchased that painting and reportedly sold it a few years ago for $150 million.