Exposed: the family secret behind Manet's most controversial picture

It was a picture that scandalised 1860s Paris and changed the course of art history. Edouard Manet's Le déjeuner sur l'herbe, with its depiction of a defiant nude lolling in a park beside two fully clothed male companions outraged the French establishment when it was first exhibited in 1863.

It was a picture that scandalised 1860s Paris and changed the course of art history. Edouard Manet's Le déjeuner sur l'herbe, with its depiction of a defiant nude lolling in a park beside two fully clothed male companions outraged the French establishment when it was first exhibited in 1863.

But the scandal might have run even deeper if the Parisian haute monde had realised the truth about Manet's private life - that his father, one of France's most esteemed judges, had an illegitimate son whom the painter brought up as his own, a new documentary claims.

Auguste Manet, a senior judge at the Palais de Justice and a holder of the Légion d'Honneur, fathered a son in 1852 by the woman he hired to teach his children piano, the art critic Waldemar Januszczak says channel Five's Every Picture Tells A Story.

Ten years later, Edouard married the piano teacher, Suzanne Leenhoff, and it was widely assumed that her son, Leon, who appears in many of Manet's paintings, was the artist's own child.

In return for his part in the cover-up, Januszczak believes that Auguste was forced to accept and fund his son's vocation as an artist - a rebellious choice of profession for someone from a genteel family. "Rather surprisingly, Manet's father accepted his son's decision to become an artist and provided the money for Manet's upkeep and education," Mr Januszczak says. "It can't have been easy for Auguste Manet to sponsor and fund his son's rebellion. And it certainly wasn't in character. Perhaps Auguste wasn't as respectable as he pretended to be. Perhaps Edouard knew something about his father that his father didn't want others to know. Perhaps Manet junior had something on Manet senior.

"Very convincing circumstantial evidence points to Auguste Manet being the father of little Leon, and not Edouard. And if he was, then the Le déjeuner sur l'herbe, begun in the year of Auguste's death, would have been informed by a highly personal understanding of the shallowness of respectability, the power of lust, and the prevalence of hypocrisy."

When Edouard was born in 1832, Auguste hoped he would follow him into the law, but instead, encouraged by his uncle Charles Fournier, Manet chose to become an artist. He trained as an apprentice in the studio of Thomas Couture, who was renowned for painting classical scenes packed with nudes. In 1863, the Salon, the heart of the Paris art establishment, refused to show Le déjeuner sur l'herbe, but later that year it went on display in the Salon des Réfuses, sparking accusations that it was idiotic and childish.

Its shock value lay in Manet's break with the classical portrayal of the female nude, instead portraying a naked woman staring brazenly back at the viewer, accompanied by two men dressed not in historical garb, but fashionable outfits of the day.

Professor John House of the Courtauld Institute, says: " Le déjeuner sur l'herbe was just frankly provocative. It was obviously completely immoral for a modern-day woman to be seated naked with two fully clothed men. If you had river gods and nymphs and so on, that was absolutely fine, but not people who were absolutely, obviously, explicitly contemporary in their clothes."

The model, Victorine Meurent, remained Manet's muse for a decade, featuring in his other famously scandalous picture Olympia, depicting a naked courtesan on a couch, which was considered so shocking that pregnant women were advised not to look at it.

The two male figures in the painting are Manet's brother Gustave, wearing a fez, and his wife's brother Ferdinand. The painting is inspired by two earlier works - Concert Champêtre, which at the time was widely considered to be by Giorgione, although it has more recently been attributed to Titian, and Raphael's Judgement of Paris, both of which feature naked women beside fully clothed men.

Michael Hall, the editor of the fine art magazine Apollo, said he was unconvinced that Le déjeuner sur l'herbe was a comment on Manet's domestic situation. "I'm rather suspicious about biographical interpretations of paintings," he said.

"I would like to see further evidence that anybody would have understood the painting in that way at that time. What we do know about Manet is that he wanted to recreate the Old Masters for modern times. I think the painting is about his response to the past."

Despite the disapproval it provoked, Le déjeuner sur l'herbe marked a pivotal moment in art history, Mr Januszczak says. "Without this painting there wouldn't have been Impressionism, and without Impressionism there wouldn't have been Modern Art."

Every Picture Tells A Story, channel Five, Thursday 26 August, 7.30pm