An Oxford University professor says he has found conclusive proof that a portrait dismissed as a 19th Century forgery is a genuine Leonardo.
Art history expert Martin Kemp says he has found a 15th Century book in Poland from which the piece was taken.
The work, bought in 1998 for $21,850 (£14,000), could be worth millions.
But the National Gallery, which will stage a Da Vinci exhibition in November, said there was "no general agreement" the work was by the artist.
Martin Kemp and fellow author Pascal Cotte published a book last year, La Bella Principessa: The Story of the New Masterpiece by Leonardo Da Vinci, about the inks and coloured chalk on vellum portrait.
It was sold as an early German 19th-Century work under the name Head of a Young Girl in Profile to the Left, at Christie's New York in 1998.
Kemp and Cotte previously identified the teenager as Bianca Sforza, the daughter of Leonardo's patron Ludovico Sforza, Duke of Milan (1452-1508), and his mistress Bernardina de Corradis.
They also concluded that three stitch holes in the left margin of the work made it likely the portrait had previously been bound in a book.
The author now says the portrait was taken from a volume, produced specifically for the marriage of Bianca Sforza and the duke's commander, Galeazzo Sanseverino, in 1496.
The Sforziad, by Giovanni Simonetta, printed on vellum, is now housed in Warsaw's National Library.
Kemp and Cotte say they have demonstrated "that a corresponding page has been removed at some unknown date from the Warsaw book, and that the vellum of the portrait closely matches in all respects the physical characteristics of the opening pages".
The stitch holes in the vellum of the portrait match those in the book, they add.
The pair had previously stated that a fingerprint found on La Bella Principessa was similar to a fingerprint on a Leonardo work in the Vatican.
They also found "significant" stylistic parallels with Leonardo's Portrait of a Woman in Profile, which hangs in Windsor Castle.
And they said they had established that the drawing and hatching were made by a left-handed artist, as Leonardo is known to have been.
"We can now be confident that the portrait portrays Bianca, in celebration of her marriage, and that Leonardo was the artist," Kemp and Cotte said in a statement this week.
However, critics of the theory have said there are no other Leonardo works on vellum and that the painting is not consistent with the artist's style.
The National Gallery in London's exhibition, Leonardo Da Vinci: Painter at the Court of Milan, will run from 9 November to 5 February.