Italian police are investigating the possible discovery of a long-missing Gustav Klimt painting that was found inside a wall of the gallery where it disappeared from 23 years ago.

The missing Klimt is valued at €60 million ($66 million).

Klimt's Portrait of a Lady, reportedly stolen in 1997, was found on Tuesday by a gardener clearing up ivy on an exterior wall of the Ricci Oddi modern art gallery, in the northern city of Piacenza, reports the Guardian. The gardener came upon a metal panel which opened to hold the painting in a bag.

Experts are examining the Klimt for authenticity, but stamps and wax on the verso seem to initially indicate that it is the original 1917 work of the Austrian art nouveau painter.

“If the findings confirm the authenticity of the painting, it would be a sensational discovery and we would be ready to exhibit it in the gallery as early as January,” said Jonathan Papamerenghi, a member of the Piacenza council with responsibility for culture. “We are talking about the most sought-after stolen painting in the world after Caravaggio’s Nativity with St Francis and St Lawrence.”

The Caravaggio was stolen in 1969, and is among the world's greatest artworks still missing, along with a Vermeer, Rembrandt's only seascape and other priceless works taken from Boston's Gardner Museum in 1990. The Art Loss Register (ALR) database is one service that consists of over 500,000 cultural objects and works that have been reported missing worldwide.