A painting found hidden in an Italian gallery in December is an authentic Gustav Klimt piece stolen almost 23 years ago, experts have confirmed.

The Portrait of a Lady was one of the world’s most sought-after stolen artworks before it was found concealed in a wall of the Ricci Oddi modern art gallery, the same gallery from where it went missing in the northern city of Piacenza.

Ornella Chicca, the Piacenza prosecutor, said at a press conference: “It’s with no small emotion that I can tell you that the work is authentic.”

The painting was discovered by two gardeners as they cleared ivy on an exterior wall of the gallery on 10 December. The pair discovered a metal panel, which, when opened, revealed a cavity with a painting in a bag. An initial inspection indicated that the painting was the 1917 work by the Austrian art nouveau painter before two experts were appointment by the prosecutor to confirm its authenticity.

In a further twist, Ermanno Mariani, a journalist with the Piacenza newspaper La Libertà, received a letter from two people claiming to have stolen the painting before hiding it in the wall.

“A mystery upon a mystery for which the investigations are ongoing,” La Libertà reported on Friday. “The certification of its authenticity opens the door to the investigators’ work and it cannot be ruled out that the name of a suspect might soon appear.”

The theft of Portrait of a Lady was discovered on the morning of 22 February 1997 but police believed it had been removed three days earlier. Investigators at the time suspected an inside job. The investigation was reopened in 2016 following the discovery of DNA traces of a thief on the painting’s abandoned frame.

Police believe the thieves used a fishing line to hook the masterpiece off the wall and haul it up through an open skylight to the roof of the gallery, where the frame was discarded.

The Klimt is considered particularly important 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.

Patrizia Barbieri, the mayor of Piacenza, said the news of its authenticity “is of historic importance for the artistic and cultural community and for the city of Piacenza”.

Jonathan Papamerenghi, the city’s culture chief, said in December that if the painting’s authenticity was confirmed then they would be ready to exhibit it in the gallery as early as January. At the time, he said the piece was the most sought-after stolen painting in the world after Caravaggio’s Nativity with St Francis and St Lawrence.

The Caravaggio painting was stolen in 1969 from the Oratory of San Lorenzo in Palermo and its theft is considered one of the greatest mysteries in art history.