The Virgin on the Rocks has hung at the gallery for 117 years but, unknown to its millions of admirers, it concealed a second work that cannot be seen with the naked eye.

When curators examined the painting using near infra-red scanning technology they found the first new Leonardo work for more than 70 years.

The scanner, which uses technology designed to spot tanks on a battlefield, picked up not only the underdrawing for The Virgin on the Rocks but also a completely different picture.

Beneath the Virgin’s head is a hand, clutched to a woman’s breast. As curators uncovered more of the drawing it became clear that the hand belonged to a second Virgin Mary gazing down at a series of lines that are believed to be the beginnings of a baby Jesus.

The drawing is likely to be a depiction of Mary kneeling before Jesus, a scene painted by many Renaissance artists and known as the Adoration of the Christ Child.

The hidden head is remarkable because it is the same head that appears — in reverse — as that of St Philip in Leonardo’s The Last Supper.

Luke Syson, a curator at the gallery, said that Leonardo commonly used drawings of male models to paint pictures of women. The original sketch of the model survives in the Royal Collection and bears a strong resemblance to the third disciple to the right of Christ in The Last Supper. “He was happy to use (the drawing) for the Virgin Mary,” Mr Syson said. “He was used to using male models for pictures of female beauty. We know it was for The Last Supper but it seems he revised that head for the Virgin in our picture.”

It is the first discovery of a large Leonardo work since the 1930s, when the Ginevra de’ Benci and the Madonna of the Carnation were attributed to the artist. An unseen painting by Leonardo has not been discovered since some of his wall paintings were uncovered in Milan in 1894.

Mr Syson said that he was astonished when he saw the new images. “It was a most amazing moment when we turned the camera on to the head of the Virgin and saw that the lower part of the head was a hand.”

The Virgin of the Rocks covers two preparatory drawings. The first, drawn on to the wood backing, appears to be an Adoration of the Christ Child. The second, drawn on top of a priming layer, is the underdrawing for the visible painting.

Mr Syson believes that Leonardo changed his mind while completing a commission for the Milanese Confraternity of the Immaculate Conception, a religious order.

The confraternity commissioned a painting from Leonardo in 1483. The artist complied but appears to have been so impressed by his work that he demanded a bonus. The confraternity refused, so Leonardo sold the painting — now held by the Louvre — to the Duke of Milan.

The confraternity appears to have then asked the artist to paint another picture. Mr Syson said that Leonardo began a different painting but, for personal reasons or because the confraternity demanded it, stopped and made a copy of his previous work. Mr Syson added that Leonardo might also have made a copy because he did not have time to create a new work.

The discovery of the second underdrawing, the basis for The Virgin of the Rocks, also proves that the painting is not merely a copy of the one that hangs in the Louvre. The drawings show that the artist experimented with details such as the size of his subjects’ foreheads, applying what he had learnt about the way light falls and the anatomy of human beings. The discovery of the new picture will be celebrated by those who subscribe to conspiracy theories propounded in The Da Vinci Code, Dan Brown’s bestselling novel that suggests that one of the characters in The Last Supper is Mary Magdalene.

“I fear we are going to have a wave of fans descending on the gallery,” Mr Syson said. “We can confidently expect some pretty insane predictions.”

UNDER THE SURFACE

• A sleazy nightclub scene was found beneath the sombre landscape of Picasso’s La Rue de Montmartre in 2004
  
  
• A Rembrandt self-portrait was hidden for more than 300 years after an inferior artist painted over it. Dutch conservators took nine years to restore the work, which went on display in Amsterdam in 2003
  
  
• A £100 picture of peasant women fetched £367,000 at a Japanese auction on being authenticated as a Van Gogh
  
  
• Erotic drawings by L.S. Lowry were found beneath pictures after his death

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