An art auction opened on Sunday for an early Renaissance painting by the Italian master Cimabue, just weeks after the rare work was discovered in an unsuspecting elderly Frenchwoman’s kitchen.

Christ Mocked, by the 13th-century artist also known as Cenni di Pepo, has been valued at €4 to €6 million ($4.4 to $6.7 million) for what could be France’s biggest art sale of the year.

The tiny unsigned work, just 26cm by 20cm, was found to be in excellent condition, though covered in grime from having been displayed right above a cooking hotplate.

Art experts at Turquin in Paris used infrared reflectography to confirm that the piece is part of a larger diptych from 1280, when Cimabue painted eight scenes of the passion and crucifixion of Christ. Each of the two panels in the diptych had four scenes.

The Christ Mocked painting’s French owner in Compiegne, northeast of Paris, thought it was just an old religious icon when she sought an appraisal from auctioneers at Acteon, the house organising the sale in nearby Senlis.

“The style, the ornamentation of the gold background, the matchings of the backs of the panels and the overall conditions of the three confirms that they were part of the left side of the same diptych,” Acteon said.

Only two other elements of the diptych are known to exist – The Flagellation of Christ displayed at the Frick in New York, and The Virgin and Child with Two Angels at the National Gallery in London.

The Virgin was valued at £6.5 million ($8.3 million) when it was given to the museum in 2000 in lieu of inheritance taxes by a British aristocrat who found it while cleaning out his ancestral seat in Suffolk.

Cimabue is renowned for his mosaics, frescoes and altarpieces.

Historians say only about a dozen works on wood – all unsigned – are thought to have been made by the Italian artist.

His more natural and nuanced depictions marked a turning point for Italian painters still influenced by highly stylised Byzantine art.

Art historians consider him a trailblazer for the creative Renaissance that would flourish under greats like Giotto, one of Cimabue’s pupils.

Christ Mocked has been on public display at the city hall in Senlis since Friday.