The Isles of Scilly Wildlife Trust has spoken out after discovering that people are starting fires within the grounds of the old church on St Helen's.
The Trust’s Rangers found the remnants of a fire pit inside the scheduled ancient monument on the island.
They wrote on social media: “By moving stones around, building structures like this and making fires within our ancient monuments you are actually breaking the law.
“If anybody witnesses this type of behaviour please do say something. If you are unable to say something directly to the individuals and encourage them to do it differently, please do report it.”
Isles of Scilly Police said the people responsible are likely to be unaware of the implications of their actions and requested that all residents, seasonal workers and visitors share the importance of preserving the uninhabited islands for archaeological and conservation purposes.