One of the art world's biggest mysteries has been solved by a chance discovery inside the walls an Italian art gallery.

A Gustav Klimt original that was stolen nearly 23 years ago was found by a gardener in December while clearing ivy from the walls of Ricci Oddi Modern Art Gallery in Piacenza.

The artwork was inside a bag, hidden in a hole in the wall.

Portrait of a Lady went missing from the gallery during renovation work in February 1997.

Italian authorities have confirmed the painting, that has an estimated value of at least €60m ($66m; £51m), to be genuine.

"It is with no small emotion that I can tell you the work is authentic," said Prosecutor Ornella Chicca.

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.

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

Portrait of a Lady is a later work by art nouveau master Klimt, who painted it between 1916 and 1917.

It was purchased by Giuseppe Ricci Oddi in 1925 and kept in the gallery until it was stolen on February 22, 1997 amid preparations for a special exhibition.

The painting's frame was left on the roof, in an attempt to fool investigators into thinking thieves had broken in through the skylight. However the skylight's dimensions would have made it impossible for the painting to fit through.

Last month, 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.

Painting found inside Italian gallery wall confirmed as stolen Klimt_1
Inset: Two forensic police officers at the metal panel in which the painting was found. Photo: AP

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

Why the painting was left in the wall remains a mystery.

The Klimt is of particular significance because shortly before its disappearance an art student realised it had been painted over another work previously believed lost – a portrait of a young lady that had not been seen since 1912 – making it the only "double" Klimt known to the art world.

The condition of the artwork is "relatively good," Mr Cauzzi said. "It's gone through a few ordeals but only needs some routine care, nothing particularly complicated."

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, said Ms Chicca.

Once the tests are complete the artwork will go back on display.