Is this the earliest work by Leonardo?

An American scholar has claimed that some details in Verrocchio's 'Tobias and the Angel' are by Leonardo da Vinci. Vanessa Thorpe reports

He was the apprentice a master painter might long for, and dread at the same time. A dedicated pupil, determined to learn every lesson and absorb every influence, he would grow up to eclipse not just his teacher, but all his contemporaries.

When the 14-year-old Leonardo da Vinci arrived in Florence in the late 1460s to start his apprenticeship at Andrea del Verrocchio's renowned studio, an unstoppable process was set in train. From that point, with each brushstroke he made, Leonardo began to develop the inquisitive sensibility that would later produce the most important creative works of the age.

Not much is firmly known about his early years. Generations of art historians have argued about just how much Leonardo got up to in Verrocchio's studio. How, they have wondered, was it possible for Leonardo to burst on to the scene in his twenties with his individual artistic intelligence already almost fully formed? In repeated attempts to chart the learning curve of the young genius, details from various Verrocchio paintings have been isolated for inspection. Do they bear the classic Leonardo trademarks, the swirling brushwork and the almost Impressionist concern with realism?

Now, following 10 years of research, David Alan Brown, Curator of Italian Renaissance Painting at the National Gallery of Art in Washington DC, is about to put forward what he believes is startling and conclusive proof that Leonardo was artistically mature and active at a much earlier date than has previously been thought. In a bold piece of scholarship, he will claim that the painting Tobias and the Angel, which hangs in the Sainsbury Wing of the National Gallery in London and is attributed to Verrocchio, was in fact a joint enterprise with Leonardo.

"It was like a revelation," says Dr Brown, speaking to the Independent on Sunday from Washington. "Then I kept going back to look at the picture again and again. And each time I felt more and more that I was right. Leonardo must have been doing something over these years - and here it was. His time in Florence was a sort of gestation."

In his forthcoming book, Leonardo da Vinci: Origins of a Genius, Brown establishes the case for believing that the dog, the fish and even the head of the figure of Tobias are in truth the work of the young Leonardo and not of his esteemed teacher. If this is true, Tobias and the Angel becomes the earliest Leonardo work yet discovered.

"Leonardo's share in the London picture - the animals and some, if not all, of the figure of Tobias - is his earliest identifiable accomplishment as a painter," says Brown. He reckons these sections of the panel reveal the work of a mind "at once more rational and imaginative than Verrocchio's".

Tobias and the Angel belongs to a popular tradition of representations of well-known allegories. The image of the boy and his guiding angel was familiar in 15th-century Florence, because the city's legion of superstitious merchants habitually commissioned paintings of the young Tobias before they sent their sons abroad on business.

A little dog often featured in the scene, but it's the fish the boy carries which is really significant. Its potent entrails are being conveyed back home in the small metal box held by the angel so Tobias can use them to cure his father's blindness.

In the London Tobias and the Angel, the two small animals have clearly been added after the background had been painted, giving the dog in particular a ghostly, rather comical aspect. It was this insubstantial creature's curly coat that alerted Brown to his discovery. "The dog's coat is strikingly similar to the long rippling hair of Leonardo's angel in the Baptism," he argues, referring to the later painting by Verrocchio which is now widely acknowledged to show the handiwork of the young Leonardo. "Indeed, the terrier's and the angel's curls are almost interchangeable," he continues. "The dog also anticipates the oversized ermine in Leonardo's later Portrait of a Lady in Krakow; both animals, however realist in form and action, are interpreted in the same unmistakably personal idiom."

It would have been quite routine for apprentices working in a maestro's studio to execute some of the smaller details of a major work. Brown, though, thinks this case was exceptional. He suspects that Leonardo actually got to paint most of the figure of Tobias.

In later life, Leonardo is on record as saying that he did not approve of painters using real gold in their pictures. Instead, he favoured the use of a yellow ochre that looks like gold, and which is used in this panel to decorate the clothes of the boy. The angel's robes, in contrast, are illuminated with gold leaf. Brown also considers that the brushstrokes used to pick out the sleeves of the two characters are further evidence of Leonardo's hand. "While Verrocchio painted the sumptuous fabric [of the angel] as woven, his pupil's brushstrokes describe highlights on gold threads, not threads per se."

Tobias's head, he claims, may be by Leonardo too. It is silhouetted, he points out, in exactly the way that Gabriel's profile appears in front of a tree in the 1470s Annunciation. Also, the highlighted strands of the boy's hair,he argues, contrast with those of the angel, and result in a quiff-like forelock that was to become a kind of Leonardo signature. Brown deduces that "to achieve this effect, Leonardo seems to have experimented with left-handed brushstrokes in the hair above the ear. When viewed under a microscope, certain of the hatchings here taper upwards to the left with the movement of the artist's wrist."

Finally, and most importantly, Brown uses his discoveries to put forward a new theory. He now believes that Leonardo must have helped with several of the surviving plastic works from the Verrocchio production line: for example, some of the terracotta heads of women and children which have not yet been conclusively attributed.

Brown is going to explain his revelations in a talk he will give first at the National Gallery of Art in Washington in September, and then at the National Gallery in London in October. His claims are bound to cause controversy. The National Gallery already knows much of the detail of his argument and is reacting with caution. Curators at the gallery concede that Brown's theory is "interesting" and worthy of greater consideration. But they are not planning to reattribute Tobias. So, for the moment at least, Verrocchio can keep his glory.

Does that annoy Brown? Apparently not. Coolly academic, he says he understands his fellow curators' collective reserve. "I am not at all surprised by this reaction. A cautious and judicious response is called for. We are dealing with a very young Leonardo. It is a phase that has never been explored before, and the portions of the work that I identify as Leonardo's are close enough to Verrocchio's style to have been missed as his work until now."

'Leonardo da Vinci: Origins of a Genius' (Yale University Press, pounds 35) will be published on 27 August.