Da Vinci's 'Virgin' is decoded

For 125 years, The Virgin of the Rocks by Leonardo da Vinci has hung in the National Gallery in London, often dismissed as a copy of another painting owned by the Louvre.

But eight months of high-tech investigations have discovered hitherto unsuspected drawings underneath, proving the artist set out not to copy his own masterpiece but to paint quite a different work altogether.

The discovery of the new Leonardos has thrilled curators and scientists who were expecting to find only the type of preparatory sketches revealed by detailed studies of other works.

Luke Syson, the National Gallery's curator of early Italian painting, spokeyesterday of when he and researcher Rachel Billinge realised they were not going to find what they had expected. "I just realised that a picture that had seemed utterly familiar, so well studied, was going to reveal extraordinary secrets. It was an amazing moment."

Leonardo was originally commissioned to create The Virgin of the Rocks in 1483 by the Milanese Confraternity of the Immaculate Conception – a group of church lay people – for their oratory altarpiece. However, the finished work was so fine, he demanded a substantial bonus. When they refused, he sold the work, probably to the Duke of Milan.

"What seems likely is that the confraternity came back to him because they still had a hole in the middle of the altarpiece and said, 'We do want the picture,'" Mr Syson said.

"But the new discovery seems to demonstrate that when Leonardo was asked for a substitute picture, he initially thought he would start a different painting and not simply a version. But at each stage, he's got pulled back to something closer to the Louvre version."

Mr Syson suspects it was Leonardo's paymasters, the Milanese confraternity, who rejected his innovations and insisted on their own version of the work they had originally expected to get. "I have a vision," he said, "of Leonardo saying 'I'm doing this beautiful new painting', but them saying: 'We want one like the other one."

It was finally installed in the chapel for which it was intended in 1508.

Mr Syson said the National Gallery had set out to investigate the relationship between its own, not quite finished, painting and the work in the Louvre which is thought to be more fully the hand of Leonardo. The respective dates of the two works is one of the hottest disputes in art history.

But he and Ms Billinge could scarcely contain their excitement when the infra-red reflectography, using carbon in the lines of the drawings, revealed an image of a hand where the face of the finished Virgin was.

The full underdrawing shows a kneeling figure with downcast gaze and pious gestures.