For five centuries, it has been hidden among a cache of drawings by Leonardo da Vinci. The simple ink sketch of an elderly, bearded man was made by an assistant on a discarded piece of paper already used by the Renaissance master for studies of a horse's leg.
But experts have now identified the wistful-looking face as that of Leonardo himself, making it one of only two surviving portraits of the legendary artist produced during his lifetime.
The face, along with that of a handsome younger man, is believed to have been drawn by one of his studio assistants shortly before Leonardo's death 500 years ago on May 2, 1519.
A sketch of Leonardo da Vinci, c.1517–18, by an assistant of Leonardo
Later this month it will go on public display for the first time at the Queen's Gallery at Buckingham Palace as part of the biggest exhibition of Leonardo's work in more than 65 years.
The newly identified portrait will appear alongside the only other surviving contemporary image of Leonardo, drawn by his favourite pupil Francesco Melzi at around the same time.
Martin Clayton, Head of Prints and Drawings, Royal Collection Trust, who made the discovery, said: "If you compare this sketch with Francesco Melzi's portrait of Leonardo, you can see strong indications that this too is a depiction of the artist.
"The elegant, straight nose, the line of the beard rising diagonally up the cheek to the ear, a ringlet falling from the moustache at the corner of the mouth, and the long wavy hair are all exactly as Melzi showed them in his portrait.
Exhibition curator Martin Clayton
"Leonardo was renowned for his well-kept and luxuriant beard, at a time when relatively few men were bearded – though the beard was rapidly coming into fashion at this time.
"Alongside Melzi's portrait, this is the only other contemporary likeness of Leonardo." The work was discovered in a leather-bound portfolio of the artist's drawings kept after his death by Melzi and later acquired by Charles II.
Now held by the Royal Collection Trust, 200 of the drawings will form the exhibition Leonardo da Vinci: A Life in Drawing, to mark the 500th anniversary of his death.
The remarkable works include preparatory sketches for some of the celebrated painter's most famous paintings, including The Last Supper, the Virgin and Child with St Anne, the Battle of Anghiari and Salvator Mundi – which in 2017 became the most expensive painting in the world.
Visitors to the Queen's Gallery will also see Leonardo's Studies of hands for the Adoration of the Magi – on public display for the first time and only visible in ultraviolet light due to age.
Francesco Melzi's portrait of Leonardo
Giving an insight into his scientific curiosity, there are sketches of the movement of water and groundbreaking anatomical studies, including The Fetus in the Womb and the Heart and Coronary vessels There are also examples of Leonardo's engineering and cartographic work, including a map of the town of Imola, which he paced out himself, and designs for weapons and an equestrian monument.
The ink, chalk and metalpoint sketches are accompanied by Leonardo's distinctive mirror-image writing and provide an unrivalled glimpse into his thinking and wide-ranging interests, including painting, sculpture, architecture, anatomy, engineering, cartography, geology and botany.
The new exhibition follows 12 simultaneous exhibitions of 12 drawings each across the UK. Later this year a selection of 80 of the drawings will go on display at The Queen's Gallery, Edinburgh.
Leonardo da Vinci: A Life in Drawing goes on display at the Queen's Gallery, Buckingham Palace, from May 24.