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So did the auctioneers, who catalogued the oil on canvas, which was part of the collection of Ruth Stone, daughter of Samuel Klein of Klein's Department Stores, as 'Portrait of a Woman with a Feather in her hat' and attributed it to Gilbert Stuart.

Having done some research before the sale, which suggested all was not as it seemed, Mr Mould bought the painting and, upon cleaning it, found definite hints of stubble around the face.

"This fuelled further investigation that resulted in the astonishing discovery that the portrait is of the legendary spy, diplomat and transvestite Chevalier D'Eon that has been lost since 1926," he explained.

The French-born D'Eon (1728-1810) is known as the "Patron Saint of Transvestites" and the term "eonism", meaning cross-dressing and cross-sexuality, derives from him.

This is thought to be the earliest surviving formal portrait of a male transvestite, and research undertaken since by the gallery has attributed the work to the theatrical artist Thomas Stewart (b.1766), who specialised in painting actors and theatrical scenes in London in the 1790s - the same time as D'Eon was performing on stage as a fencer in drag.

The painting was almost certainly commissioned by the Earl of Moira, an 18th century Irish soldier, playwright and patron of the arts, and passed through his family till the 1850s when it was given to George III's ex-doctor - possibly as a mark of its medical curiosity. Descendants of the doctor sold it to an American collector at the beginning of last century, after which it apparently vanished.

According to Mr Mould, it is now "under serious consideration" by the National Portrait Gallery in London for an undisclosed price. Should they decide to purchase the work, it will represent the gallery's first oil painting of a cross-dresser in guise.