A long-lost painting by 17th-century Flemish master Peter Paul Rubens has been found in a Paris mansion, a French auctioneer said on Wednesday.

"It is a masterpiece," said Jean-Pierre Osenat, who had made the discovery, referring to the sketch of Jesus Christ on the cross painted in 1613.

"It was painted by Rubens at the height of his talent," he told AFP, adding that the artwork was in "very good condition".

Osenat, the head of the eponymous auction house, said he had found the painting in September 2024 while preparing to sell the private mansion in the French capital's chic 6th district.

"It is an extremely rare and incredible discovery," he told AFP.

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 72.5 centimetres (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.