The image matches that of the face on the front side of the shroud, with faint details of a nose, eyes, hair, beard and moustache.
The discovery is published today in a scientific paper by two Italian scientists. The shroud is among the most controversial Christian relics. Supporters say the handwoven cloth is the burial shroud of Jesus, and that the "image" visible on its front - the smudged outline of a man's body - is proof of the Resurrection.
But scientists who have studied the image have repeatedly said it is a convincing and clever fake, probably from medieval times, "no miracles are necessary to explain the [front] image" and it "can be explained by reference to highly probable, well-known chemical reactions".
But that assertion will meet a renewed challenge with the new research, in the peer-reviewed Journal of Optics A: Pure and Applied Optics. The "underside" of the shroud has rarely been seen. It is hidden under a piece of cloth that was sewn on by nuns in 1534 after a fire that blackened parts of the huge cloth, which measures 14ft3in by 3ft7in (4m by 1m).
The reverse side of the shroud was scrutinised in detail for the first time only in 2002, when the extra cloth was unstitched during restoration. The latest research builds on photographs taken then which were published in a book by one of the church's keepers of the shroud.
