The painting has been authenticated by German art historian Nils Buttner, known for his research on the master of the Flemish Baroque, Osenat said.
Its provenance was certified through methods including X-ray imaging and pigment analysis, he added.
Osenat remembered pacing back and forth while a committee of experts was deciding on the authenticity of the painting. Then came a phone call from Buttner.
“Jean-Pierre, we have a new Rubens!” he said Buttner had told him.
The auctioneer called the painting “a true profession of faith and a favourite subject for Rubens, a protestant who converted to Catholicism”.
“It’s the very beginning of Baroque painting, depicting a crucified Christ, isolated, luminous and standing out vividly against a dark and threatening sky,” he said.
Although Rubens produced many works for the Church, the newly discovered painting, measuring 105.5 by 72.5cm (42 by 29 inches), is likely to have been created for a private collector.
It is thought to have belonged to the 19th-century French academic painter William-Adolphe Bouguereau and later to the owners of the Parisian mansion where it was found.
The painting is set to be auctioned off on November 30.
-Agence France-Presse