The Shroud of Turin, which is held by many Roman Catholics to bear an authentic portrait of the face of Jesus, will soon come under severe scientific and historical criticism.

The shroud, allegedly the burial cloth of Christ, bears the marks of a crucified human figure and of the face of a man which became fully visible only during this century, when it was treated as a photographic negative.

Since the shroud was pubilicly displayed in Turin in 1978, there has been a widespread demand for scientific analysis of it, particularly by carbon dating, to seek to establish if it is 2,,000 years olds.

A stumbling block to shroud believers has always been the sheer size of it -- 14 feet by 3 feet. It seemed almost inconceivable that something so large could have lasted for so long when only fragments of other cloth of the period have survived.

Carbon dating would involve the destruction of a small area of the cloth, about the size of a pocket handkerchief. It has been a disappointment to scholars that Catholic authorities have refused to allow this. One explanation is that the church believes techniques are not yet sufficiently refined.

The proved history of the cloth dates back only to the 14th century, when it emerged from the crusading Templar order.

The big blow to its authenticity is the imminent publication by the micrologist, Dr. Walter MacCrone of Chicago, of evidence that he has found traces of iron oxide on it, indicating that the image was painted on, rather than being imprinted by a human form. By this reasoning, the shroud was the creation of some unknown medieval artist.

The rev. David Sox, an American Episcopal priest working in London, is the secretary of the British Society for the Shroud of Turin. His book, "The Image of the Shroud," to be published next month, is a good deal more sceptical than most on the subject.

"So far as I am concerned, if the existence of paint is proved, that is the end of the shroud as a miraculous object," he said.

"However, the fascinating question remains of who was the artist and how he operated."

Sox believes that future study of the shroud may have to concentrate on exacly how it and other medieval Christian relics were produced. In particular, he cites evidence that the negative photograph effect exists in other medieval paintings.

Some enthusiasts, for whom the shroud has become an object of almost religious faith -- "the witness of Christ for the 20th century," as some have called it -- are fighting a rearguard action. One explanation is that a medieval artist could have touched up with paint an image that already existed.