A painting found stashed inside a wall at an Italian museum has been confirmed as the stolen Portrait of a Lady by Austria's Gustav Klimt, prosecutors said on Friday, two decades after it went missing. 

"It is with no small emotion that I can tell you the work is authentic," Prosecutor Ornella Chicca told reporters, referring to the painting discovered last month by gardeners inside an external wall at the Ricci Oddi Gallery of Modern Art in Piacenza.

Museum officials had initially said they could not immediately determine whether the painting was indeed the stolen Klimt until scientific tests were performed. 

Painted between 1916 and 1917, the Expressionist work depicts the face and torso of a young woman with brown hair, over an emerald green background. 

The painting - which the museum estimated could be worth between 60 and 100 million Euro - went missing in February 1997 while the museum was closed for work. 

In December, gardeners removing ivy from a wall found a small ventilation space inside of which they discovered the painting, without a frame, wrapped in a black garbage bag.

The ivy covering the space had not been cut back for almost a decade. 

In 1996 it was determined through X-ray analysis that the painting covered up another, that of the face of a different woman.