|The haunting image of a tall, bearded man bearing the marks of crucifixion that adorns the Turin shroud has been called one of the greatest religious hoaxes - one that has intrigued scientists and believers for decades.
Yesterday, the respected Institute of Physics in London renewed speculation about the revered object by announcing that a "ghostly image" had been discovered on the back.
The cloth has been hailed by some as the burial shroud of Christ but, in a milestone study 15 years ago, three teams concluded after carbon dating that it originated from between 1260 and 1390.
It led to the widespread conclusion that the shroud was a pious hoax created for the pilgrimage business.
Since then, believers have launched a counterattack on various fronts: clinging to the notion that the lack of a convincing explanation of the image suggests a miracle; questioning the carbon dating study by claiming that it sampled cloth that had been repaired later using medieval material; that carbon dating was confused by a burst of radiation released on the resurrection; or even that God created the shroud to look like a medieval forgery.
Those who have attempted to answer the vexed question of the shroud's origins have often found themselves accused of poor science, blinkered thinking or vested interests.
So it was brave of the institute to issue a statement yesterday that said: "Lying behind the known image of the bearded man bearing the marks of crucifixion, the new image has striking three-dimensional quality and matches in form, size and position the known face."
However, it does seem to require an act of faith to see the face in an image reproduced in the paper by Giulio Fanti, professor of mechanical and thermic measurements at Padua University.
When asked whether religious conviction would help see the face of Christ, a spokesman for the institute said, somewhat defensively: "As scientific publishers, my colleagues and I are worried that we might be connected with religious/metaphysical views that we do not hold."
The supposed image uses photographs taken during a restoration two years ago.
Hidden beneath a piece of Holland cloth that was sewn by nuns in 1534, after a fire had blackened parts of it, the reverse side of the shroud was fully scrutinised for the first time in 2002, when the 14ft linen was unstitched from the Holland cloth. The back surface of the shroud was photographed in detail.
"When I saw the pictures, I was caught by the perception of a faint image on the back surface of the shroud. I thought that perhaps there was more that wasn't visible to the naked eye," said Prof Fanti.
"Though the image is very faint, features such as nose, eyes, hair, beard and moustache are visible."
However, the enhancing procedure did not uncover the full body image as it appears on the front side.
The presence of a face on both sides of the shroud would seem an obvious feature: one idea, put forward by Prof Stephen Mattingly of the University of Texas Health Science Centre in San Antonio is that the image was created by bacteria that caused a stain in the cloth, so presumably it stained both sides to some extent.
But this is not the case for the shroud, said Prof Fanti. "On both sides, the face image is superficial, involving only the outermost linen fibres. There is nothing in the middle. It is extremely difficult to make a fake with this feature."
Difficult, but not impossible.