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.