A painting said to show King Charles II's favourite mistress Nell Gwyn is being sold to help secure the future of the Kent castle where it once hung.
Portrait of a Young Lady and Child shows one of the King's mistresses
Sir Peter Lely's 17th Century masterpiece, Portrait of a Young Lady and Child as Venus and Cupid, is expected to fetch up to £2m at auction.
It is being sold to save Chiddingstone Castle, near Edenbridge, home of the late art collector Denys Bower.
A trust is selling the picture to prevent the castle's closure.
It is hoped it will then be reopened to the public.
The painting is thought to have been owned by Charles II and kept in the royal bedchamber behind a secret panel, at the Palace of Whitehall.
Lely, who was Dutch, was the official artist to the king and is considered the finest painter of the Restoration, according to Christie's spokesman Matthew Paton.
A question mark hangs over which of the king's mistresses is featured in the painting, which shows a naked woman reclining with a child at her side.
It was traditionally believed to show the king's favourite mistress, Nell Gwyn, with her son Charles Beauclerk.
However, records suggest it is more likely to be the king's former mistress, Barbara Villiers, Countess of Castlemaine and Duchess of Cleveland.
The painting has previously been catalogued as a portrait of Barbara Villiers and exhibited as a portrait of Nell Gwyn.
It will be sold at the Important Old Master and British Pictures sale at Christie's in King Street, London, on 5 July.