$100M Painting Found In Museum It Was 'Stolen' From Decades Ago

One of the world's most sought after paintings has been found stashed in a wall of the very same gallery it was supposedly taken from more than 20 years ago.

A gardener clearing overgrown ivy off a wall outside Italy's Ricci Oddi modern art gallery on Tuesday noticed a metal panel in the wall and pried it open.

Inside the nook, between the outside world and the internal gallery walls, was Gustav Klimt’s famous Portrait of a Lady wrapped in a black bag.

“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.

The painting disappeared during a brazen heist in the northern city of Piacenza in February 1997.

At the time, police believed that three thieves used a fishing line to hook the haul from the gallery wall and reel it up through an open skylight.

The frame of the painting was left on the roof, appearing to show that thieves had broken from above. But it turned out the skylight would have been too small for the painting to fit through.

A frame grab taken from a handout video made available by the Italian police. Image: AAP

Officers enlisted the help of a well-known local art thief to find the piece, but all that was found was a rip-off on the Italian-French border three months later.

It has been touted as one of the most baffling art theft cases of its kind.

The gallery is confirming the authenticity of the painting and whether it had really been stashed in the hole for decades, before it will make an official announcement, but it is believed to be the real thing.

“It would be the best Christmas present ever,” Gallery Vice President Laura Bonfanti told local media.

“Of course the work must now be examined by experts to verify its authenticity, but at first glance, based on the wax seals and stamps we saw on the back of the canvas, it appears to be the original.”

One of the world's most sought-after painting found after 20 years. Image: AAP

The 1912 piece gained global attention in 1996 after a young art student discovered it was painted over another Klimt painting which was presumed lost.

It is the only 'double' Klimt portrait known to the art world and has been valued at a staggering AU$97 million.