Far from being admired as an extraordinary genius, Leonardo da Vinci was repeatedly lampooned and teased about his unusual red hair and his unconventional sexuality by other leading artists of his day. Although the work of the great Italian was popular in his time, an extensive new study of the artist to be published this week has outlined evidence that he was the butt of gossipy jokes in Renaissance Milan.

Author Simon Hewitt has unearthed a little-studied image held in Germany, a “comic strip” design made in 1495 to illustrate a poem, that showed how Leonardo was once ridiculed. In one of its colourful images, An Allegory of Justice, a ginger-haired clerk, or court lawyer, is shown seated at a desk, mesmerised by other young men, and represents Leonardo da Vinci. “The identity of Leonardo as the red-headed scribe is totally new,” Hewitt told the Observer ahead of the publication of Leonardo da Vinci and the Book of Doom.

“The comic-strip picture is in an obscure manuscript in Berlin and has never been consulted before by any Leonardo scholar.”

The key passage in Hewitt’s book identifies the painter through a series of clues in the precious illustration. He is shown as a “left-handed clerk … with a wooden lyre at his feet: evidently a caricature of Leonardo da Vinci”. The lyre was Leonardo’s instrument and his father, Ser Piero, who is depicted resting his right arm on his shoulder, “is brandishing a sheet of paper that surely represents the anonymous document denouncing Leonardo for sodomy, deposited in a Florence tamburo in April 1476”.

Red-headedness was rare in 15th-century Milan, but not unknown, and was regarded as freakish. Hewitt suspects Leonardo was descended from the Khazars, a Turkic people from the Asian steppe who scattered around the Black Sea and eastern Europe and had red hair. “As the 13th-century Arab historian Ibn Sa’id al-Maghribi put it: ‘Their complexions are white, their eyes blue and their hair flowing and predominantly reddish.’ ”

Close study of the illuminated manuscript copy of Gaspare Visconti’s epic poem Paolo e Daria, revealed to Hewitt that Leonardo da Vinci is also likely to be the object of ridicule because of the absent-minded way he is shown to be drawing on the tablecloth, rather than on his sketch notebooks, and by his apparent fascination with a half-naked young man who is clutching “a rocket-like, Leo-invented contraption”.

“Further evidence of Leonardo’s identity, and homosexual leanings, is provided by the group of eight strapping figures alongside,” argues Hewitt, who has conducted five years of research into Leonardo and his circle in search of the truth about a controversial portrait, La Bella Principessa. Along the way, he discovered the image from Visconti’s epic poem which also features a young blonde woman holding a pair of scales. Hewitt identifies this as Bianca Sforza, the daughter of the duke of Milan and the subject of the disputed wedding portrait by Leonardo da Vinci, completed a year later.

The central argument of Hewitt’s new book is that Bianca is also the mysterious young woman in La Bella Principessa. He believes he can now prove that Leonardo really made this work. Since 2009 experts have clashed over its authenticity, with the self-confessed forger Shaun Greenhalgh claiming it was one of his.

Hewitt tells the tragic life story of Sforza, who was later murdered, and explains why Leonardo’s portrait of her was included in one of the most lavish books ever produced – the Sforzida – considered to be “a book of doom”.

Leonardo’s co-illustrator on this project, Giovan Pietro Birago, was paid even more than him. In tracking down this 15th-century evidence, Hewitt came across other illustrations made by Birago which deliberately poke fun at Leonardo. One Birago features a cherub committing sodomy. “As one of the few figures with ginger hair in Birago’s entire oeuvre, this is clearly a mischievous reference to Leonardo da Vinci,” writes Hewitt.