A long-lost Medieval masterpiece has been discovered hanging above an elderly woman’s kitchen stove in northern France. When it goes up for auction in October, the painting is estimated to fetch around €6 million (£5.3 million).

“It was considered special by the family, but they thought it was an icon,” explained Philomene Wolf, the auctioneer who found the artwork during a house clearance in Compiegne this summer.

Small and unassuming in size, the tempera painting is believed to be Christ Mocked by the Florentine master Cimabue (1240-1302). Infrared testing was used to confirm the attribution, with some experts stating there is “no disputing” its origin.

Cimabue, also known as Cenni di Pepo, was a pioneering Italian painter and is known as the father of western art. His paintings break away from the highly stylised Italo-Byzantine tradition to depict more life-like religious scenes.

Paris-based old master expert Eric Turquin has hailed the incredibly rare painting as “the only small-scale work of devotion that has been recently added to the catalogue of authentic works by Cimabue.”

Subsequent research by Turquin revealed that Christ Mocked could also be part of polyptych – a type of altarpiece painting consisting of three or more panels. Two other scenes from the set, which show the Flagellation of Christ and Madonna and Child Enthroned between Two Angels, are displayed in the Frick Collection in New York and the National Gallery in London.

“You can follow the tunnels made by the worms,” said Turquin, who compared the marks made in the panel by wood-eating larvae to those found in the other sections of Cimabue’s polyptych. “It’s the same poplar panel,” he added.

But not everyone in the art world is celebrating this extraordinary discovery. The Guardian Jonathan Jones warns that the identification of Old Masters can be fraught with peril. Whilst stumbling across a forgery is the biggest fear, general misattribution is rife in the market as well.

“What’s worrying is the way this painting, like the Caravaggio from the loft, is being boldly called a Cimabue without wider discussion,” complained Jones, referring to a painting discovered in a French attic that Turquin later attributed to Caravaggio.

Dismissing any doubts about her attribution to Cimabue, Wolf claimed “I am so lucky. I am at the beginning of my career, and you can wait an entire lifetime before making such a discovery.”

French auction house Acteon will sell the painting on 27 October.