According to tradition, the transformation of the blood of San Gennaro, preserved in two phials, protects the city from disasters such as earthquakes or an eruption of Mount Vesuvius.

Scepticism about this popular event on the religious calendar, held in Naples cathedral and witnessed by thousands of people every year, has been heard before.

This time, members of the Italian Committee for the Investigation of the Paranormal (Cicap) have said the red-coloured contents are a thixotropic substance, based on iron chloride. This means that it liquefies when stirred or vibrated and returns to solid form when left to stand. According to Cicap, the substance was probably stumbled upon by an alchemist or a painter in medieval times.

Attempts to explode the myth about Naples' much-loved patron saint has however, reignited the debate about science versus faith in Italy.

Members of Cicap, who include Umberto Eco and two winners of the Nobel Prize, have been accused of trying to undermine the religious beliefs of the dwindling numbers of the faithful. They have also been called spoilsports and compared to magicians who reveal their tricks.

The Church, as usual, has sidestepped the issue. It has never claimed that the San Gennaro event is a miracle but it has not allowed independent tests on the phials containing the blood. It did consent to an examination by a scientist appointed by the Church, who reported that he found traces of blood, but his work has never been published in any scientific journal.

Defence of the San Gennaro tradition has come from Vittorio Messori, a noted Catholic writer who is close to Pope Benedict XVI.

He says he has seen with his own eyes the blood emit tiny bubbles and change into liquid.

No single occasion when the relic is brought out is the same, he adds. Sometimes the colour is a dark red, sometimes it is lighter. One year the liquid is thin and another year it is more viscose.

Mr Messori says successive cardinals of Naples have been willing to allow independent scientists to examine the liquid but the glass phials are incredibly delicate and what would happen if they broke and the precious liquid was exposed to the air? There are many other important things for scientists to investigate, he adds.

Furthermore, he says, Catholics do not claim the liquification is a miracle, only "a sign".

Cicap is unrepentant about stirring up the controversy again. "We are accused of being blasphemous but we are not interested in people's religious beliefs," says spokesman Massimo Polidoro. "We are people who want to understand the world around us. When people have both sides of a story they can make up their own minds."

The organisation has undermined quite a few myths. Holy statues that weep tears of blood are a common occurrence in Italy, but nobody ever sees the actual flows of blood, only the stains. According to Cicap, most of the blood is found to be human, smeared on by owners of the statues after they have pricked their fingers or opened a small cut on their hands.

"In the late 1940s there were dozens and dozens of weeping Madonnas in Italy," says Mr Polidoro. "The message was that people should vote for the Catholic parties in order to defeat the Communists."

An outbreak of weeping Madonnas also occurred in 1994, he said, in support of Prime Minister Silvio Berlusconi who also promised to defend the country against Communism.

The Padre Pio phenomenon has also been investigated. The monk from southern Italy was said to have lived with stigmata, the wounds suffered by Christ on the cross, for most of his life. He was also seen flying in the air, protecting his hometown from enemy bombings during the Second World War, and apparently had the gift of bi-location - the ability to be in two places at once. He was canonized in 2002.

"There are all these legends about this guy but when you look for proof, there is none," says Mr Polidoro.

Cicap's next project is to investigate crop circles, a new phenomenon in Italy in the past two years.

"They're pretty simple at the moment, nothing like the ones you see in England. But I'm sure the people who are doing them here will catch up soon," laughs Mr Polidoro.