Jean-Honore Fragonard's The Portrait of Francois-Henri d'Harcourt will star in Bonhams' sale of the renowned collection of Dr Gustav Rau on December 5.
The masterpiece is considered one of the finest in the 18th century French artist's series of fantasy portraits, and with Fragonard considered a leading exponent of the rococo style, it is sure to sell well.
The work is especially rare as its sitter can be identified. Many of Fragonard's fantasy portraits are personifications of the arts, rather than impressions of actual people.
It will form the centrepiece of one of the first sales to be held at Bonhams' new £30m London saleroom, which is due to open in October.
"There are always those rare occasions when the work of an artist who is widely regarded as a genius appears on the market. In the world of Old Master Paintings, however, it is almost unique nowadays to sell one of the very masterpieces for which that artist acquired his reputation," explained Bonhams' director of old master paintings, Andrew McKenzie.
"As is the case with truly great art, Fragonard here transcends the constraints of his own age to create a tour de force that holds its own today alongside the finest achievements of modern civilisation."
Also starring in the December auction will be a 15th century depiction of the crucifixion by an unknown German artist, which holds a £1.5m ($2.3m) high estimate, and a portrait of the Duchess of Montebello by Pierre-Paul Prud'hon, which is valued at £1m (1.5m).
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