A British art historian says he’s identified a lost da Vinci portrait that was ripped out of a 15th century book, the Guardian reports. If this is true, Christie’s auction house sold the drawing years ago for about 100 million British pounds less than it was worth.

Oxford University emeritus professor of art history Martin Kemp claims that the portrait is a picture of Bianca, the daughter of the Duke of Milan, Ludovico Sforza, who was also a patron of Leonardo da Vinci, the Guardian reports.

He started investigating the origins of the portrait, called “La Principessa,” in 2008 after noting that it was made by a left-handed artist, LiveScience reports. Da Vinci was the only left-handed artist at the duke’s residence at the time.

"The chance of identifying the vellum book it came from was pretty small, a needle in the haystack, one would say," Kemp told LiveScience. However, following up a tip from a University of California at Berkeley art historian, Kemp found a missing page in a book of Sforza family history in Poland.

According to LiveScience:

Upon examination, Kemp saw that the stitch holes from the page match up with the stitching on the book, but they aren't the only evidence Kemp puts forward. Because vellum is made from processed skins, each sheet has different qualities. The thickness and composition of this sheet matches up perfectly with the vellum from the book, Kemp said.

Many art historians disagree that Kemp’s findings prove the picture is a da Vinci. One issue is the lack of other Leonardo works on vellum, the Guardian reports.

"We still believe that it is not an authentic drawing by Leonardo," Verena Dahlitz of the Abertina art gallery in Vienna, which passed on exhibiting the drawing, told LiveScience. "We think that the drawing is from the 19th century."

A Christie’s spokesman told the Guardian: "The attribution of the majority of all Old Master paintings and drawings is determined through comprehensive scholarship and science. This work continues to prompt a healthy debate among the world's leading scholars."