Livestock are escaping through damaged fencing

The damaged areas are at East Dartmoor Woods and Heaths National Nature Reserve, near Bovey Tracey.

They include two Sites of Special Scientific Interest (SSSIs) and a Scheduled Ancient Monument (SAM).

English Nature, Dartmoor National Park, English Heritage and the Woodlands Trust say the culprits must be stopped.

An inconsiderate few are putting others at risk and ruining their enjoyment of irreplaceable sites James Mason, Woodland Trust

English Nature says much of the damage is deliberate and it is working with police to tackle the problem.

Spokesman Phil Page said: "Off-road 4X4 drivers and scramblers are deliberately causing damage to sites.

"They have cut through barbed wire fences and smashed wooden barriers."

The conservation group says the damage is also causing problems for local farmers, with cattle on Trendlebere Down Common escaping through the gaps being made.

There are fears that if the illegal activities are not stopped, damage to the Iron Age Hill Fort in Houndtor Wood could be disastrous.

Prosecution warning

Debbie Griffiths, from Dartmoor National Park, said: "Earthwork sites are particularly vulnerable to damage through erosion, and valuable information about both the monuments and Dartmoor's past could be lost for ever."

Any person causing damage to an SSSI without "reasonable excuse" can be prosecuted and, if found guilty, fined up to �20,000 in a magistrates court or an unlimited fine at crown court.

The Woodlands Trust says the illegal activities must be stopped immediately.

Woodland Officer James Mason said: "People need to be aware that what they're doing is dangerous, illegal and is damaging the sites.