Was the science that declared the Turin Shroud a fake flawed? Ian Wilson believes he has uncovered evidence that renders its findings dubious.

By all normal standards of rationality the Turin Shroud - with its famous "photographic" imprint of a crucified man said to be Jesus Christ - ought not to be a subject of any further interest. Back in 1988 radiocarbon-dating tests by three state-of-the-art scientific laboratories "conclusively" dated its linen to between 1260 and 1390, seemingly confirming a medieval French bishop's allegation that the shroud had been "cunningly painted" sometime around the 1350s. The Catholic Church formally accepted that the shroud was a fake.

Yet when the shroud goes on public display in Turin Cathedral on April 18 (its first such showing in 20 years) there can be little doubt that the atmosphere will be reverent and the queues long, as if this cloth really did once wrap and become imprinted with Jesus's crucified body. Furthermore, the Catholic Church has promised a repeat showing in the year 2000, in honour of the 2,000th anniversary of Jesus's birth. So is this cynical pandering on the part of the church to those millions of its adherents still given to superstitious relicry? Or could there, even now, be a case for the shroud to be taken seriously?

Back in 1988 the "fake" verdict was widely regarded as the definitive arbiter on the shroud, and the laboratory scientists involved played up this idea to the hilt. With their still innovative Accelerator Mass Spectometry method of carbon dating in direct competition with the more traditional method, careers depended on how well the AMS would stand up to public scrutiny. The Oxford laboratory's founding director, Edward Hall, told journalists: "There will be some flat-earthers who won't accept this. They're on to a loser," authoritatively adding: "There was a multi-million-pound business in making forgeries during the 14th century. Someone just got a bit of linen, faked it up and flogged it."

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The odd aspect is that, in the ten years since the shroud was carbon dated, no one has come up with any satisfactory explanation for how someone, either back in the Middle Ages or at any other time, actually could have "faked it up", as Professor Hall suggests. There have been two independent theories, one by Surrey physician Michael Straiton, the other by authors Christopher Knight and Robert Lomas, that some crusader was crucified in mockery of the crucifixion of Jesus, thereby creating the image that we now see. There is the "Leonardo da Vinci did it as the world's first photograph" theory. There is also Chicago microscopist Walter McCrone's "an artist hand-painted a photographic negative by sheer chance" argument.

But all these theories have serious difficulties. McCrone has no satisfactory answer to how an artist not only uniquely produced a photographic negative, but also embellished this with bloodstains that have convinced a galaxy of modern-day pathologists. The crucified crusader hypothesis falters because no normal dead body, even a crucified one, produces an imprint of the type that is evident on the shroud - added to which there is incontrovertible historical evidence, from well before the parameters of the 1260-1390 radiocarbon dates, of the existence of a shroud-like cloth of Jesus bearing just such an imprint. As for the Leonardo school, its proponents' reliance on the say-so of one pseudonymous "Giovanni", a putative member of the Priory of Sion, who were chosen to release his society's secret to the world, is pretty unconvincing.

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But even if the means by which the shroud could have been faked back in the Middle Ages remain unaccounted for, and even if there is still the ghost of a chance that the shroud might genuinely date back to the first century ad, there remains one seemingly insuperable obstacle: how could three state-of-the-art radiocarbon-dating laboratories, one in the US, one in the UK and one in Switzerland, all make an error as huge as 1,300 years?

What needs to be understood is that carbon dating, despite its undeniable value as an archaeological aid, is not as reliable as its quoted "margins of error" suggest. Take, for instance, the case of one of Britain's most famous archaeological discoveries in recent years, Lindow Man, a sacrificial victim unearthed from a Cheshire peat bog in 1983. The Atomic Energy Research Establishment's radiocarbon-dating laboratory at Harwell dated him to the fifth century ad, Oxford University's carbon-dating laboratory dated him to the first century bc and the British Museum's carbon-dating laboratory dated him to the third century bc. Yet each laboratory claimed its date accurate to within 100 years. There has never been any satisfactory resolution to this discrepancy, each individual laboratory resolutely sticking to its particular findings, with archaeologists left baffled on the sidelines.

Then there is the case of the Manchester Museum's ancient Egyptian mummy no.1770, which, under the direction of Egyptologist Rosalie David, received arguably the most exhaustive scientific examination that has ever been conducted upon any ancient Egyptian, royal or commoner. When Dr David sent for radiocarbon-dating samples taken both from the mummy cadaver and from its wrappings, the result that came back was that the wrappings were purportedly a thousand years younger than the mummy. As she recognised, although it was just conceivable that a young woman had been rewrapped a thousand years after her first interment, it was unlikely. Yet the only alternative was that there was something about the wrappings, of linen, just like the shroud, which had interfered with the carbon-dating reading, making the wrappings appear much younger than they actually were.

For years there seemed no satisfactory explanation for this until a chance discovery by Texas physician Leoncio Garza-Valdes. An authority on ancient Mayan jades, Valdes's pride was dented when, on presenting two specimens at a conference, two experts from New York told him that they were modern fakes, the give-away being the jades' "new" and shiny-looking surface. Thoroughly unsettled, Valdes examined the surface under his microscope, noting, to his surprise, that instead of the sheen deriving from some kind of hand-polishing, as he had previously assumed, what was actually responsible was a natural clear plastic coating, built up by millions of tiny micro-organisms, rather in the manner of a coral reef.

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This was still no proof that the jade was genuinely of ancient workmanship, but Valdes's second surprise came when samples of blood, smeared on the jade during Mayan rituals, were carbon dated. For while the reading was ad c.400, thereby showing that the jade was genuinely ancient, even this date was several centuries younger than the artefacts should have been, judging by their artistic style. So, could the presence of this coating, which had covered the blood as well as the jade proper, have interfered with the dating? If so might it explain anomalous datings of other archaeological artefacts, such as mummy 1770 - and the shroud?

Intrigued, in 1993 Valdes travelled to Turin where Giovanni Riggi, the microanalyst who had cut off the piece of shroud used for the carbon dating in 1988, allowed him to view under the microscope small portions that had been held back from the laboratories. To Valdes's satisfaction he saw these specimens were covered with a similar coating to that which he had observed on the Mayan jade. The coating was composed of partly still living organisms accumulated to such a thickness in proportion to the linen that it would have had a major effect on the dating.

However convincing this might appear to Valdes, could he persuade others that this could be the key to the shroud having been seriously misdated? He showed the coating to Stephen Mattingly, head of the University of Texas Health Science Center's microbiology department in San Antonio, then sought the opinion of other key individuals, including Rosalie David, and Harry Gove, the inventor of the AMS method used to date the shroud. In 1996 all four jointly decided to conduct an experiment with an Egyptian mummy that had never before been radiocarbon dated. They chose a sacred ibis, unlikely to have been rewrapped in antiquity. Examining the ibis's wrappings under the microscope, Valdes observed that these indeed bore a coating, though not as thick as that on the shroud. On the basis of this he predicted that if the wrappings produced a dating of the order of 500 years younger than the ibis cadaver, then the coating would have been responsible.

In the event the carbon-dating reading showed the ibis's wrappings to be an average of 550 years younger than the ibis corpse. So Mummy 1770 had not been just some chance misdating. The presence of a bioplastic coating was demonstrably responsible for skewing carbon-dating readings. Arguably exactly the same skewing had occurred in the case of the shroud - the error in the case of the latter being particularly large because the carbon-dating sample was taken from a corner of the shroud repeatedly handled.

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For me, a convincing feature of Valdes's findings is that I observed the coating myself, without realising what I was looking at, when I viewed the shroud at first hand, without covering glass, back in November 1973. To my astonishment the Shroud seemed covered in a damask-like surface sheen. At the time that jarred with me because it made the shroud seem too new and shiny-looking. Now I regard it quite differently.

Even so it would as yet be irresponsible to claim that Valdes's bioplastic coating discovery has proven the shroud dating to be out by 1,300 years. What is needed first is the development and testing of a totally satisfactory method of eliminating this coating from the general run of archaeological samples affected by it. Then the time will be right for a fresh carbon dating - which will require every bit as much tortuous diplomacy as was needed for that of 1988. In the meantime that so "conclusive" verdict of fake that the carbon-dating laboratories delivered back in 1988 is beginning to look increasingly premature. Even for the most rational, the shroud mystery may be considered resurrected for at least a while longer.

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Ian Wilson's first book on the shroud was published in 1978. His new one, The Blood and the Shroud, is published by Weidenfeld and Nicolson next week, Pounds 20.