Sixty bags of rubbish have been dumped and set alight at a nature reserve in west Dorset.
Dorset Wildlife Trust said the fly-tipping incident at Powerstock Common on Wednesday evening was a "senseless act of vandalism".
Trust chief executive Brian Bleese said: "This is the worst case of fly-tipping we have seen on our nature reserves in a long time."
The area was made safe by Dorset and Wiltshire Fire and Rescue Service.
A fire and rescue service spokesman said: "Any fire in the countryside has the potential to be devastating, particularly on nature reserves and areas rich with wildlife."
According to the wildlife trust, Powerstock Common is one of the best places to see butterflies such as the marbled white and marsh fritillary, and is home to an array of birds including the willow warbler, chiffchaff and the great spotted woodpecker.
Mr Bleese said: "This senseless act of vandalism not only makes this wonderful nature reserve unsightly and unattractive to visitors, it is also a major drain on our resources to clear up, could be costly to remove, and, most concerning of all, poses a real threat to wildlife."