Pictured left is the way she looks today. Even those opposed to restoration agree that the layer of varnish has darkened to such an extent that colours beneath the surface have been obscured. But restorers warn that touching the painting risks losing subtle detail. Such is its fragility that the minute tonal shifts, the modelling and the shading - around the nose and eyes, for example - could vanish with the varnish.
Michael Daley, of ArtWatch, the conservation watchdog body, said: 'If shading and modelling around the mouth is lost, it is irreplaceable. No restorer can match the artist. To risk damaging that smile is a terrifying prospect.'
Jacques Franck, a French art historian who learnt of the restoration plans, said any cleaning could destroy sensitive parts in the flesh zones, less than a millimetre thick. He has studied Leonardo's sfumato technique, which with the most delicate brushwork achieved the subtle blending of light and shade and contours.
He explained that Leonardo used to finish his paintings with an extremely fine glazing and that when the protective, finishing varnish was applied, pigments in the glazing 'migrated' into the finishing varnish. He said: 'How do you remove the varnish without removing the pigment?'
Ironically, the fading is nothing new. In 1625, a visitor to the French court observed how the dress of the Mona Lisa was 'covered so badly with a certain varnish that it cannot be observed too well'. He continued: 'In spite of all the harm the picture has suffered, the face and the hands are so beautiful that whoever looks at it is brought under its spell.' Plus ca change.
Illustration: Jonathan Anstee
Leading article, page 18