King Charles II's mistress Nell Gwyn by Simon Verelst (NPG/PA Wire)

A daring portrait of King Charles II’s mistress Nell Gwyn which has not been seen for decades will go on show as part of a new exhibition opening this month.

The painting, by Simon Verelst, which shows Gwyn with her breasts exposed, dates back to around 1680 and was altered at some stage so she was covered up before it was restored.

Gwyn made her name as an actress before becoming Charles IIs mistress in 1669.

Professor Gill Perry, who is curating The First Actresses: Nell Gwyn To Sarah Siddons at the National Portrait Gallery in central London, said: Images such as this rarely seen portrait have contributed to the idea of Nell Gwyn as an early celebrity, whose life story and appearance are known through biographies and salacious gossip.

But she was a shrewd manipulator of her own public image, known not just for her affairs and outspoken views, but also for her acting abilities and famous wit.

The painting, which is owned privately and has been in the same family since the 1940s, will be on show from Thursday.