Art curators have uncovered a new Leonardo Da Vinci drawing hidden beneath the surface of one of the Renaissance artist's most celebrated works.
Da Vinci painted two versions of "The Virgin of the Rocks" between 1483 and 1508, but the picture held in London has long been regarded as an inferior copy of the original in the Louvre in Paris.
National Gallery curators in London - researching how Da Vinci copied his original - found an uncompleted drawing while using infrared scanning to see through layers of paint on the London picture.
They discovered two levels of underdrawing: one for "The Virgin of the Rocks", and another beneath for a different picture, showing the Virgin with outstretched arms.
“You can never call this a straightforward copy again because Leonardo clearly wanted to start something new,“ National Gallery curator Luke Syson told BBC radio.
Da Vinci was commissioned in 1483 to paint "The Virgin of the Rocks" by a religious order, the Milanese Confraternity of the Immaculate Conception, for a chapel altarpiece.
The 1.9 metre-tall painting shows the Virgin Mary kneeling in a cave with the infant Christ and Saint John. Da Vinci demanded a huge bonus after finishing the painting, but this was refused and so he sold it elsewhere, with the work eventually reaching the Louvre.
Some years after refusing the bonus, the Confraternity asked Da Vinci for a replacement - the version now in London - which was installed in their chapel in 1508.
The curators' detective work revealed that after receiving the second commission, Da Vinci began a new work for the Milan order, depicting the Virgin Mary adoring the infant Christ.
Mr Syson believes the Confraternity rejected Da Vinci's proposed composition and demanded a copy of their original commission. "I suspect he was forced to abandon this new very beautiful idea," Mr Syson said. "In a way it's a terrible pity."