A painting of a naked, reclining woman, long described as being Nell Gwyn, the mistress of Charles II, is now thought to depict the King’s earlier mistress, Barbara Villiers, famed as a great beauty at court.

Considered to be a masterpiece by Peter Lely, official artist to Charles II, the painting is believed to have been concealed behind a secret sliding panel for the monarch’s private enjoyment in the royal bedchamber at the Palace of Whitehall.

Documentary evidence suggests that he had the daring image covered with a landscape painting – a “sliding piece” designed by Hendrik Danckerts, which is recorded in King James II’s inventory of 1688, three years after Charles II’s death.

Research undertaken by Christie’s, which will be auctioning the painting with an estimate of £2 million on July 5 – has thrown new light on the sitter. John Stainton, head of early British pictures at Christie’s, has carried out a meticulous study that involved comparing her features with those in other portraits of Villiers – including another depiction by Lely for which she kept her clothes on.

Although recorded in the James II inventory as “Madam Gwyn’s naked portrait, with a cupid”, the sitter’s identity has long been the subject of debate, alternating between Gwyn and Villiers. While it was sold in 1955 as Villiers, in 1972 the National Portrait Gallery thought that it was Gwyn, then, in 1978, labelled it as “Portrait of a Lady and Child as Venus and Cupid”. The art collector who owned it until his death in 1977 was among those who were convinced it was Gwyn.

Mr Stainton suggested that the private manner in which such a work would have had to be kept may account for the confusion over the identity of the sitter.

Villiers – the Countess of Castlemaine and Duchess of Cleveland (1640-1709) – was already married when she met Charles. Her dominance at court in the first decade of his reign was noted by Samuel Pepys, the diarist, who observed how she “commands the King as much as ever, and hath and doth what she will”.

When Charles’s new Queen, Catherine of Braganza, arrived in England in 1662, Villiers appeared determined to maintain her position as his mistress.

Her children by the King were one of her great advantages over the childless Queen.

Charles was said to be “mighty kind to his bastard children”, visiting them often in her apartments, according to Pepys.

But her preeminence gradually diminished. Pepys recorded her pregnancy and insistence that the King own the child – “or she will bring it into Whitehall gallery and dash the brains of it out before the King’s face”.

The sale is expected to break the current record for a work by Lely, which was set last year at £960,000. Lely (1618-1680) is considered to be the finest painter of the Restoration. Born in the Netherlands, he moved to England in the early 1640s and became the natural successor to Anthony van Dyck (1599-1641), finding success portraying both Charles I and Oliver Cromwell.

In 1661 he was appointed principal painter to Charles II and the following years heralded a new artistic age, with the pleasure-loving court of the King at its epicentre. The painting is being sold at Christie’s by the trustees of the Denys Eyre Bower Bequest. Bower was a prolific collector.

In 1955 he acquired Chiddingstone Castle, a magnificent early 19th-century building near the National Trust-owned Chiddingstone village in Kent, and filled the property with his extensive and eclectic collection of art and antiques.

When he died in 1977, he bequeathed the castle and its collections to the National Trust which declined the gift becvause of the lack of an endowment fund made it financially unviable. Since 1985, the castle has been run by an independent charitable trust.

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