New scientific techniques have uncovered evidence that this picture is a previously unrecognised work by Leonardo da Vinci.

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Is this 13 x 9in (33 x 24cm) portrait, in chalk, pen and ink on vellum, mounted on an oak board, a long-lost work by Leonardo da Vinci? That is the claim being made by Martin Kemp, Emeritus Professor of History of Art at Oxford University.

Catalogued as "German, early 19th century" and sold for $19,000 at Christie's New York in the late 1990s, new scientific techniques have uncovered evidence that has convinced a growing number of the world's leading Leonardo scholars that it is a previously unrecognised work.

ATG have had exclusive access to that scientific evidence and can reveal that it literally reveals the hand - and fingerprint - of the artist in the work.

The fingerprint is "highly comparable" to one on a Leonardo work in the Vatican.

Professor Kemp's assertion is backed by scientific evidence obtained by the revolutionary "multispectral" camera pioneered by Lumière Technology of Paris.

Peter Paul Biro, the Montreal-based forensic art expert, examined the multispectral images and found a fingerprint near the top left of the work, corresponding to the tip of the index or middle-finger, and "highly comparable" to a fingerprint on Leonardo's St Jerome in the Vatican (which, stresses Biro, is an early work from a time when Leonardo is not known to have employed assistants).

A palm-print in the chalk on the sitter's neck "is also consistent in application to Leonardo's use of his hands in creating texture and shading", adds Biro, who is credited with pioneering fingerprint studies to help resolve authentication and attribution issues of works of art.

The Lumière camera has already been used to analyse Leonardo's Mona Lisa and Lady with an Ermine; by the Kröller-Müller, Van Gogh and Cleveland Art Museums; and by the Art Institute of Chicago.

Multispectral analysis reveals each successive layer of colour, and enables the pigments and pigment mixtures of each pixel to be identified without taking physical samples.

For the vellum portrait, Lumière have been able to establish the composition of the materials used in both the original drawing and the restoration. It transpires, for instance, that the green of the girl's costume was obtained by applying progressive strokes of black chalk to the yellowish surface of the vellum.

Lumière have identified the chalk as amphelite, a fine-grained black argillite (clay slate). Meanwhile flesh tints, and the amber tone of the iris, were achieved by leaving the vellum uncovered.

Infrared analysis reveals significant pentimenti throughout, with stylistic parallels to those in Leonardo's Portrait of a Woman in Profile in Windsor Castle; and shows that the drawing and hatching were made by a left-handed artist (as Leonardo is famously known to have been), whereas restoration was carried out right-handed.

There is no other known work by Leonardo on vellum, although Professor Kemp (citing a passage in Leonardo's Ligny Memorandum) points out that, when French court painter Jean Perréal visited Milan with Charles VIII in 1494, Leonardo quizzed him about the technique of using coloured chalks on vellum.

Professor Kemp suggests that Leonardo used vellum here because the portrait was intended to adorn a book of poetry in honour of the sitter; three needle holes along the left edge of the vellum indicate it was once bound in a manuscript.

The sitter's costume and elaborate hairstyle reflect Milanese fashion of the late 15th century. Carbon-14 analysis of the vellum, carried out by the Institute for Particle Physics in Zurich, is consistent with such a dating [it gave a date-range of 1440-1650].

But who is the wistful, peach-skinned, flaxen-haired teenager?

After originally code-naming her La Bella Milanese, Professor Kemp - who dubs her profile "subtle to an inexpressible degree" - upgraded her to La Bella Principessa after identifying her, "by a process of elimination", as Bianca Sforza, daughter of Ludovico Sforza, Duke of Milan (1452-1508), and his mistress Bernardina de Corradis.

Kemp believes the portrait must date from around 1496 when, aged 13 or 14, Bianca married the Duke's army captain, Galeazzo Sanseverino (a patron of Leonardo's). Tragically, she died four months after the wedding.

This would be Leonardo's first known Sforza 'princess' portrait, although he painted two of the Duke's mistresses: Cecilia Gallerani (Lady with an Ermine in the Czartoryski Museum, Cracov); and Lucrezia Crivelli (La Belle Ferronière in the Louvre).

After centuries of oblivion, the portrait resurfaced at Christie's New York on January 30, 1998, as lot 402 in an Old Master Drawings (part II) sale as a Young Girl in Profile in Renaissance Dress - catalogued as "German, early 19th century", with a $12,000-16,000 estimate.

It sold for $19,000 (hammer) to New York dealer Kate Ganz, who sold it (for about the same sum) to the Canadian-born, Europe-based connoisseur Peter Silverman in 2007.

Ganz had suggested the portrait "may have been made by a German artist studying in Italy… based on paintings by Leonardo da Vinci".

Silverman, an underbidder at Christie's sale, had other ideas and mentioned the work to Dr Nicholas Turner, formerly Keeper of Prints & Drawings at the British Museum, when he bumped into him at the Polidoro da Caravaggio exhibition at the Louvre in January 2008.

Turner, who had seen a transparency of the work a few months earlier, told Silverman he suspected Leonardo's involvement because of the "very high quality of the work overall, and the left-handed shading - his signature feature", and directed Silverman to the renowned Leonardo specialist Martin Kemp.

Professor Kemp's first reaction was that "it all sounded too good to be true - after 40 years in the Leonardo business, I thought I'd seen it all!" But, as he pursued his research, "all the bits fell into place like a well-made piece of furniture. All the drawers slotted in".

Silverman is coy about the work's current ownership, and the portrait has yet to be shown in public since its reattribution. However, Professor Kemp has recently completed a 200-page book about it (so far unpublished) in conjunction with Lumière Technology's Pascal Cotte.

Attempts to display La Bella in a museum are said to have faltered because of financial concerns linked to insurance - as a Leonardo, the portrait has been valued by London dealer Simon Dickinson at £100m.

The portrait is now due to go on display next March at a show called And There Was Light: The Masters of the Renaissance Seen in a New Light to be held in the Eriksbergshallen, Gothenburg.

The exhibition's artistic director is Alessandro Vezzosi, Director of the Museo Ideale in Vinci, Leonardo's home town, and the first man to publish the portrait as a Leonardo in his book Leonardo Infinito last year.

Professor Vezzosi is one of a growing roster of Italian art historians who believe the portrait is an autograph work, including Mina Gregori, Professor Emerita of the Florence University and President of the Fondazione Longhi; Dr Cristina Geddo, an expert on Leonardo's Milanese followers; and Professor Claudio Strinati, Head of the City of Rome Museums, who states that "the portrait constitutes a valuable addition to Leonardo's oeuvre".

To Professor Carlo Pedretti, head of the Fondazione Pedretti for Leonardo studies and widely considered the doyen of Leonardo da Vinci expertise, "this could be the most important discovery since the early 19th century re-establishment of the Lady with the Ermine as a genuine Leonardo".