Charles Geneviève Louis Auguste André Timothée d'Éon de Beaumont, to give her full name, is one of the most important transvestites in history. She was "a fascinating and inspirational figure", said Lucy Peltz, the gallery's curator of 18th-century portraits.

"We are absolutely delighted to be able to acquire this portrait. D'Eon is a particularly fascinating and important figure from 18th-century British history."

The painting was discovered by the London dealer Philip Mould at a provincial sale outside New York last year. It was being mistakenly sold as a portrait of an unknown woman by Gilbert Stuart, most famous for painting George Washington on the dollar bill.

"Even in its dirty state it was quite clear that this woman had stubble," said Mould, who bought it, brought it to the UK and began further research and restoration.

"Cleaning is always a revelation and on this occasion it revealed that not only was it in lovely condition but, more pertinently, the Gilbert Stuart signature cleaned off revealing the name Thomas Stewart, a theatrical painter working in London in the 1780s and 1790s."

Everything then began to click into place. "What is so unusual about this portrait is that it is so brazenly demonstrative in a period when you don't normally get that type of alternative persona expressed in portraiture," said Mould. There is no attempt to soften his physiognomy – basically, he was a bloke in a dress with a hat."

The discovery was tremendously exciting, said Mould. "We are the main dealers in British portraiture, doing it for something like 30 years and I must have sold two or three thousand British portraits to museums and institutions – but never have I come across something quite so idiosyncratic. I've never had anything which is so off-beam."