Bologna: Italian detectives may be on the cusp of solving one of the art world's most baffling mysteries after what appears to be a Gustav Klimt painting that was stolen more than two decades ago was found stashed within the walls of the gallery where it went missing.

A gardener clearing ivy from the Ricci Oddi modern art gallery in the northern city of Piacenza this week saw a metal panel in the wall and prised it open to peer between the external and internal gallery walls.

A gardener had been cleaning ivy from the gallery's walls when he came upon the painting's hiding place. Credit:AP

"I found this box inside a black sack and at first I thought it was trash, but then I called my superiors right away," the worker told Italian media.

Gallery officials said they are cautiously optimistic that the painting found in the sack is Klimt's Portrait of a Lady, which went missing from the gallery in February 1997.