The extraordinary find was made by chance, when Louvre staff unhooked Leonardo's "The Virgin and Child with Saint Anne" from the museum wall as part of a broad programme of study and restoration of paintings by Leonardo, including the "Mona Lisa".
"When the work, which is painted on wood, was unhooked, a curator noticed two barely visible drawings on the back of the painting, showing a horse's head and half a skull," the museum said.
It was such an astonishing discovery that other Louvre staff present at the time could not believe it and initially said the marks on the wood must be stains.
"The Virgin and Child with Saint Anne" was painted in the early 1500s and no one had previously noticed the drawings -- at least not to the knowledge of the Louvre.
After the initial find, the museum conducted detailed tests on the back of the painting. Photographs taken with an infrared camera revealed that there were not two but three drawings. The third one is of a Child Jesus playing with a lamb."This is an exceptional discovery because drawings on the back of paintings are very rare and no example by Leonardo was previously known," the Louvre said.
It said the drawings recalled some of Leonardo's known works and suggested that the child and lamb could have been sketches for the painting on the other side of the piece of wood. "The style of the drawings recalls the style of Leonardo, but research is ongoing to clarify their authorship," it said.