It was a ménage à trois that thrilled early 19th century society: Vice-Admiral Horatio Lord Nelson's affair with actress Lady Emma Hamilton, wife of elderly aristocrat Sir William Hamilton.
It was understandably a popular subject for newspaper gossip columns and cartoonists, but in the days before photography and paparazzi, there had, to date, been no image of them together.
Now, a small watercolour sketch, painted from life during an 1800 tour of the Mediterranean has been confirmed as the only 'live footage' of the couple.
Nelson, in his blue livery with gold epaulettes, is distinctive in his bicorn hat worn sideways. He is seen holding a parasol over Lady Hamilton in his left hand (he had lost his right hand in a battle in Tenerife three years earlier). The figure to his right has been identified as Sir William.
The painting dates to the year before Lady Hamilton gave birth to Nelson's daughter Horatia, and five years before his death at the Battle of Trafalgar.
Other sketches in the book feature naval ships, crew members on the Foudroyant's deck and various locations around the Mediterranean.
The artist later fell out with Lady Hamilton, after she was advised to keep her distance from the Nelson/Hamilton set for fear it would harm her reputation.
Ménage à trois: Nelson is distinctive in his bicorn hat worn sideways. He is seen holding a parasol over Lady Hamilton in his left hand. The figure to his right has been identified as Sir William
Lady Hamilton, was furious, and wrote of Knight: 'Altho she is clever and learned, she is dirty, ill-bred, ungrateful, bad mannered, false and deceitful. But my heart takes a noble vengeance I forgive her...'
The sketchbook, which had been in the possession of a naval family, is now due to be auctioned at Bonhams in London on March 23, along with three letters signed by Nelson.
Simon Roberts, from Bonhams, said: 'It's quite remarkable. There were of course cartoons of Nelson and Lady Hamilton but nothing done from life.
'The sketchbook is an evocative little thing and Ellis Knight was not a bad artist. She has a track record and some of her paintings have come up for sale in their own right.
'There are 37 sketches in the book which are numbered and indexed. It is an authentic document, like a visual diary.'
The sketchbook, which measures 7.5x5in, is estimated to sell for up to £15,000.