Every year Neapolitans pack into the city’s cathedral to witness the “miracle”. In an atmosphere nearing hysteria, the archbishop holds up a glass phial that is said to contain the dried blood of San Gennaro, the city’s patron saint, and declares that it has liquefied.

Liquefaction, a central part of life in Naples, is held to guarantee good fortune. By contrast, its failure is thought to presage disasters ranging from the eruption of Mount Vesuvius to the defeat of Napoli football club.

But yesterday’s ceremony, which marked the 1,700th anniversary of the martyrdom of San Gennaro, was overshadowed by a claim made by an Italian scientist that the liquefaction was nothing but a fake.

Margherita Hack, an astrophysicist, said: “There is nothing mystical about this. You can make the so-called blood in your kitchen at home.”

Professor Hack and fellow scientists at the Italian Association for the Study of the Paranormal said that the phial contained “an iron-based chemical compound dating from medieval times”.

The dark brown gel was solid until shaken , when it liquefied. Professor Hack said that the compound was hydrated iron oxide, or FeO (OH), which had the characteristics of blood.

Her report has caused outrage in Naples. Marchese Pierluigi Sanfelice, an aristocrat who is one of the official guardians of the phial and takes part in the liquefaction ceremony, said that the Church had conducted tests on the phial in the 1980s which showed that its contents included haemoglobin, the key pigment in blood corpuscles.

The Marchese said: “The trouble with scientists is that when they cannot find an explanation they invent one. They simply cannot accept that some things are beyond human understanding.”

Marisa Laurito, an actress from Naples, said that Professor Hack “should leave the miracle of San Gennaro well alone. It doesn’t do any harm, and prayers to the saint do a lot of good”.

She added: “It is hard for outsiders to understand, but he is the very soul of Naples; our symbol.”

Rosa Russo Jervolino, the Mayor of Naples, who has led the fight to rid the city of its high crime rate and encourage tourism, said that yesterday’s successful liquefaction was “a sign that San Gennaro is still protecting our city, a strong sign of hope and an encouragement for everyone to work for the common good”.

The phial is kept in a safe in Naples Cathedral and taken to the high altar amid prayers and invocations. The announcement of the liquefaction is greeted with a 21-gun salute at the 13th-century Castel Nuovo. The ceremony takes place in May and December as well as on San Gennaro’s feast day in September. The first recorded reference to the “miracle of the blood” was in 1389. Disasters which are reputed to have struck when the blood failed to liquefy include the plague of 1527, in which tens of thousands died, and the earthquake in southern Italy in 1980, that killed 3,000.

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