DA VINCI CODE fever is threatening the fabric of Rosslyn Chapel, the Scottish church reputed to be the resting place of the holy grail and featured in the bestselling book.

Fears that the building and its unique stone carvings are being put at risk by an influx of visitors will lead to tourists being banned from making unsupervised visits to the medieval chapel.

Since The Da Vinci Code was published in 2003, visitor numbers at the 15th-century chapel have risen by 56%. Last year almost 70,000 visited the building in Midlothian, making it one of the country’s most popular tourist attractions.

Senior heritage figures fear the influx of tourists could lead to a build-up of humidity in the small chapel, causing further damage to its already fragile foundations and eroding its sandstone carvings.

The problem of overcrowding is most severe in the summer months. Last July alone more than 9,000 people visited the chapel, a 96% increase over the previous year.

Fractures have already appeared in the chapel’s walls as a result of movement of the structure over the past 20 years and a £3m renovation project is due to start later this year.

This summer the Rosslyn Chapel Trust will introduce hourly guided tours to prevent damage to the delicate carvings — including its famous Apprentice Pillar. At peak times, the chapel will be closed to anyone not on a guided tour.

It will be the first time visitors have been denied unfettered access to the chapel, which was founded in 1446.

To reduce pressure on the church and occupy those waiting for a guided tour, medieval gardens and an expanded visitor centre are planned. A full survey of the foundations is also to be carried out.

“In recent months, with visitor numbers rising by unprecedented amount, we have had discussions about what can be done to minimise these conservation risks,” said Stuart Beattie, project manager of the Rosslyn Chapel Trust.

“There would be less of a problem if we could spread the number of people evenly over 12 months but the fact is that we have a huge influx of visitors in the summer months.

“We do welcome the extra income, which helps us fund our restoration project. But there is a danger that so many people brushing against the sandstone features repeatedly could have a damaging effect.

“There is also the concern that this number of people could really ruin the visitor experience. Many people come here for a spiritual or illuminating experience, not to walk around with their arms clamped by their sides.”

Brian Park, of Page and Park architects, which is working on the restoration of the chapel, said humidity was the principal threat. An air humidity monitoring system will be installed to monitor the situation.

Rosslyn Chapel was founded in 1446 by Sir William St Clair, the last St Clair prince of Orkney, and has been linked to biblical and Masonic legend throughout its history.

In The Da Vinci Code, an investigation into the murder of an agent of an ancient clandestine society leads to Rosslyn Chapel. The book has sold more than 17m copies.

Interest in the chapel is likely to increase even further next year with the launch of a Hollywood film based on the book. Tom Hanks and Harrison Ford have been approached to star in the movie.

Nick Bridgeland, inspector of ancient monuments at Historic Scotland, said it was vital that the building was protected from the “Da Vinci effect”.

“This is a completely unique building that has baffled architectural historians for centuries,” he said. “It is a great treasure and it deserves the best protection we can give it.”

Times Recommends

• Time-worn Freddie gives up Test matches
• Brighton company 'involved in cyber attack'
• The art of survival: it's all in the mind