Mountain bikers in Somerset have been told to stop using a protected nature reserve as a downhill track.
The county's wildlife trust said bikers were damaging Gilling Down Nature Reserve near Compton Dundon - a Site of Special Scientific Interest (SSSI).
The area is known locally as the Red Hills due to exposed escarpment of red and grey bedrock.
The trust said despite putting out more warning signs there had been a "sharp increase" in damage by mountain bikers.
Emma Daniel, from Somerset Wildlife Trust, said: "It is becoming increasingly serious as the bike tyres are starting to rip through the very thin turf opening up new areas of erosion, as well as increasing erosion on exposed soft and crumbly bedrock.
"The reserve is a SSSI for the unimproved limestone grassland which supports 36 notable plants and animal species, some of which are legally protected scheduled species."
She added that the mountain bikers were also breaking the law under the Wildlife and Countryside Act which states that it is illegal to "cause damage to or destruction of any structure or place used by a scheduled animal for shelter or protection".
