An ancient hill fort dubbed "one of the jewels in the crown" of the South Downs National Park has been damaged, police have said.
Illegal metal detecting is believed to be behind the disturbance to the ground at the 5,000-year-old Cissbury Ring site near Worthing in West Sussex.
Described by the National Trust as the most historic hill on the South Downs, its ditch and ramparts enclose some 65 acres and it is a habitat for butterflies, flowers and rare plants.
The damage caused at the largest hill fort in Sussex, which police have said is "irreversible", has provoked outrage in the metal detecting community.
Sussex Police has now launched an investigation as the site is protected by the Ancient Monuments and Archaeological Areas Act 1979.
Paul Roberts, Historic England's inspector of ancient monuments for Kent, Sussex and Surrey, said: "Irresponsible and criminal metal detecting destroys evidence of our national story that belongs to us all.
"It is a great shame that Cissbury Ring has now been damaged by the selfish action of a few."
Police said illegal metal detecting was a "shady, unscrupulous act", and they have appealed for information.
PCSO Daryl Holter, Sussex Police's heritage crime officer, said: "It is unlikely we will know if items were removed, but any such interference is simply stealing our past and robbing us of the opportunity to interpret and understand it."
Metal detecting is only ever allowed on National Trust land under a special licence. The National Trust has appealed to the public to report sightings of illegal activity to the police.
National Trust archaeologist Tom Dommett said: "We are saddened to see this intrusion at such an important archaeological site, and the potential loss of priceless and irreplaceable information about our past."
Anyone with information about the damage is asked to call Sussex Police on 101, or Crimestoppers anonymously on 0800 555 111.