The mystery of the religious decorations that lie underneath the high street of a medieval market town may never be solved if insect infestation cannot be halted

Mysterious carvings inside a hidden cavern linked to the Knights Templar are in danger of disappearing before their riddle is solved. Having survived more than 700 years, the religious decorations in the ancient cave at Royston, Hertfordshire, are under attack from an infestation of worms eating the chalk walls behind them.

The beehive-shaped chamber was hewn out of a 180ft-thick seam of chalk and extends 30ft beneath the centre of the market town, underneath a betting shop. It was uncovered by chance during building work in 1742 and the depictions of biblical scenes and portraits of Christian martyrs inside it have puzzled historians ever since.

"Some scholars believe many of the carvings depict characters revered by the Knights Templar – warrior monks who protected pilgrims on their journeys to the Holy Land in the 12th and 13th centuries," James Robinson, the custodian of the cave, told Cornerstone magazine this month.

The shape of the cave is believed to be modelled on the Church of the Holy Sepulchre in Jerusalem and its strange carvings include depictions of Christian martyrs worshipped by the templars, such as St Katherine. The knights had a special reverence for this saint as it was on 25 November, St Katherine's Day, that they won a victory over the Saracen leader, Saladin, in 1177. Other engravings on the walls of the cavern show St Christopher and St George.

Other images in the cave, such as the Sheela Na Gig fertility symbol, suggest that its origins may stretch further back to the days when Royston was a staging point on the the Icknield Way – a track that was ancient when the Romans arrived in Britain.

The site is regarded as one of Britain's least-known medieval treasures, remaining obscure because it is one of the few British locations with a templar connection that does not appear in Dan Brown's thriller The Da Vinci Code.

"It is hugely important that we find ways of halting the threat to this extraordinary ancient structure," said Philip Venning, secretary of the Society for Protection of Ancient Buildings. "Having survived for so long, it would be tragic if these fragile carvings were lost to future generations."

The porous nature of the chalk walls makes them prone to damp and flooding, while old sewage leaks have leeched into the walls and softened the chalk. This provides nourishment for the worms which feed on decayed matter and then excrete the chalk. Over the past five years some of the detail has been destroyed, leaving a honeycomb appearance on parts of the wall.

"People are astounded that such a fascinating and mysterious site can exist in such an ordinary setting as the high street of a small town," says Robinson. "Beneath the modern facade of Ladbrokes betting shop and the main road lies a unique document of this country's history."

The only other example of a similar cave is at Sloup, in the Czech Republic.