Experts are warning that hordes of tourists are removing stone work, hymn books and other fittings, and could even carve their initials in the walls of churches featured in the novel. Buildings such as the Temple Church and Westminster Abbey, which are inundated with visitors clutching copies of the novel, have already complained of thefts and vandalism. With the release of the film, currently in production, the churches are braced for another huge increase in sightseers.
Now the trustees at Rosslyn Chapel in Midlothian, Scotland, where the book suggests the Holy Grail could be buried, fear that a surge in tourist numbers could cause a rise in humidity within the chapel, irreparably damaging the unique sandstone carvings. The trustees have also discussed concerns of graffiti and theft of pieces of stone from the chapel.
The Rosslyn Chapel Trust has allowed filming to take place at the chapel next month, but Judith Fisken, an expert and former curator of Rosslyn Chapel, said: "The headache will not simply be crowd control and concern of footfall through the building. It will be souvenir hunters removing pieces of stone, taking rubbings, carving their initials and generally leaving litter."
Experts fear the problems at Rosslyn and elsewhere will rise when the film version of the book is released next year. Tom Hanks plays the part of Professor Robert Langdon who, along with Sophie Neveu, played by Amélie star Audrey Tautou, attempt to solve a murder which takes them on a journey from the Louvre in Paris to Rosslyn Chapel on a quest for the Holy Grail. The novel claims that Jesus Christ married Mary Magdalene and had a baby, starting a bloodline that exists today.
Catholic leaders have angrily dismissed the novel's theories. Westminster Abbey, which has refused to allow the film-makers access, has condemned it as "theologically unsound".
The novel has become the best-selling adult novel of all time. As a result, visitor numbers at the main venues used in the book have soared. But the rise in visitors has also led to problems. The Rosslyn Chapel Trust has been forced to instal a new entrance and triple the size of its car park as visitor numbers have boomed in the wake of the novel's success.
Temple Church in central London has already had run-ins with devoted fans determined to come away with a souvenir. More than 30 hymn books inscribed with the church's name have been stolen in the past 12 months alone. The church has yet to decide whether to allow the production company to film there.
Unlike other prominent venues featured in the novel, Westminster Abbey has not had serious problems with Da Vinci Code fans running amok. The book claims that visitors are able to do brass rubbings at the abbey, something that is not true. However, a spokeswoman for the abbey said they had "a millennium" of experience in dealing with souvenir hunters. "Since the Middle Ages we have had people trying to take bits of the abbey," a spokeswoman said. "We have learned how to combat them."
Following the abbey's refusal to allow filming to take place, Lincoln Cathedral has offered its services. Filming at the cathedral will begin this week.
Fears of the harm that Da Vinci Code fans can cause are not just confined to Britain or even to churches featured in the book. At Rennes-le-Château in France, visitors have damaged the entrance to the church, scrambling to find clues believed to be hidden in the grave of a priest, Abbé Bérenger Saunière.
12th-century Crusader church, London
Hidden away in the Inns of Court, the Temple church plays a key role as the hunt for the grail reaches a conclusion. Already well-known for its unusual "round church" design and nine, life-sized effigies of knights, visitor numbers have now soared, forcing the church to stage Da Vinci tours and leading, it is said, to the theft of hymn books by Dan Brown enthusiasts. The effigies which lie on the church floor are also at risk. The church is still uncertain about allowing filming to take place there, fearing a further influx of sightseers.
Unique 15th-century structure, Edinburgh
Long regarded as one of Britain's most mysterious buildings and rich in symbolism, Rosslyn dominates the closing chapters. Its key roles in the book and forthcoming film help to explain the extraordinary weight of visitors. Numbers have tripled since 2000 and will top 100,000 for the first time this year, leading experts to warn of irreparable damage to the delicate sandstone carvings. Increased humidity, erosion and outright vandalism are the main threats - not to mention the extra noise and litter.
Ninth-century church, French Pyrenees
An established part of the Da Vinci trail, even though it does not appear in the book. The church has long been linked with the Holy Grail theories at the heart of The Da Vinci Code, and has recently seen a surge of tourists. Its most famous priest, Abbé Saunière, shares a name with one of Dan Brown's characters. In the 1970s grail hunters used explosives to blast holes in the walls. The new generation is more respectful, but locals complain that the 120,000 visitors are causing damage once again. They have exhumed and reburied Saunière in a secret location.