Vandals have targeted the ancient Rollright Stones' site for the second time since September.
Overnight on Saturday, the volunteers' office at the site near Chipping Norton was broken into and a fire started.
The vandals turned on a portable gas heater and placed it up against woodern furniture, which started a small fire.
Pc Rob Kelly said: "We are becoming increasingly concerned about the welfare of this historically important site.
"The Rollright Stones site has been targeted several times over the last year.
"Whatever the motivation behind these attacks, what it boils down to is wanton criminal damage and we are doing all we can to stop it; and we need the public's help in this."
On September 8 and 9, vandals daubed swastikas on the hut, set fire to one of the stones in the circle and damaged a 1930s information plaque next to the King's Stone.
And in April 2004, the circle of 70 stones, which is believed to be 3,500 years old, was splattered with yellow paint, causing an estimated £50,000 ddamage and destroying historic lichen which grow on the stones.
Anyone with information relating to the latest attack is asked to contact Pc Kelly on 08458 505505 or Crimestoppers on 0800 555111.
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