The painting, which has only recently come to light, was not hidden away in a dusty attic, but was on display on the hotel walls in its famous Coco Chanel Suite. It was identified by art consultant Joseph Friedman, who was advising the Ritz on their current renovation, and fellow consultant Wanda Tymowska.
When they examined the painting they discovered the inscription C L B F, which they realised could stand for Charles Le Brun Fecit, and the date 1647. It was then shown to Le Brun experts in the French museum world and has been recognised as an unrecorded autograph work.
The Ritz archives have not revealed how the painting arrived at the hotel and it could have been there when the building of 1705 was acquired by César Ritz in 1898. The 5ft 10in x 4ft 4in (1.8 x 1.3m) oil on canvas depicts a classical subject, The Sacrifice of Polyxena, daughter of King Priam.
The painting has been consigned for auction at Christie's and will come up in their Old Masters and 19th century sale in Paris on April 15, estimated at €300,000-500,000.