A painting discovered by chance last month is a Gustav Klimt original that was stolen nearly 23 years ago, Italian authorities have confirmed.
The painting, Portrait of a Lady, was taken from a gallery in the northern city of Piacenza in 1997.
It was thought to have disappeared for good until gardeners clearing away ivy found it concealed in an external wall at the same gallery.
The Klimt has an estimated value of at least €60m ($66m; £51m).
Why the painting was left in the wall at the Ricci Oddi Gallery of Modern Art is still a mystery.
“It is with no small emotion that I can tell you the work is authentic,” said Prosecutor Ornella Chicca.
She said further tests would clarify whether the painting had been inside the wall space ever since it was stolen, or if it was placed there later.
After those tests were complete the artwork would go back on display, Ms Chicca added.
To determine its authenticity, experts studied the painting under infrared and ultraviolet light and compared the images to those taken during tests in 1996.
“The correspondence between the images allowed us to determine that it’s definitely the original painting,” art expert Guido Cauzzi said.
He said the condition of the work was “relatively good”, adding: “It’s gone through a few ordeals but only needs some routine care, nothing particularly complicated.”
Portrait of a Lady was painted in 1916-17 by Viennese artist Gustav Klimt towards the end of his life.
It was bought by Giuseppe Ricci Oddi in 1925 and kept in the gallery until it was stolen on 22 February 1997 amid preparations for a special exhibition.
The frame of the painting was discarded on the roof of the building to make it appear that thieves had broken in through the skylight. That was not the case as the skylight was too small for the painting to fit through.
In December, gardeners clearing ivy from a wall stumbled on a metal panel. Behind it lay a recess, within which was a black bag containing the missing painting.
The ivy covering the space had not been cut back for almost a decade, officials said.
Shortly before it was stolen, art student Claudia Maga revealed that it had been painted over another Klimt painting, Portrait of a Young Lady, which had not been seen since 1912.
She managed to prove her theory by persuading the Piacenza gallery’s former director to have it X-rayed.
The original painting was of a young girl from Vienna who had died. Klimt had painted over the portrait when the girl died suddenly, to forget the pain of her death.