The chase coincided with the start of an inquiry this week into his mother's death in a 1997 car crash after being pursued by photographers.
British newspapers on Friday ran pictures of William and Kate Middleton, both 25, in a car after leaving a London nightclub in the early hours. "Threatening" paparazzi then chased the couple in their car, Paddy Harverson said.
"Having already been photographed leaving the club, he and Kate Middleton were then pursued in his car by photographers on motorcycles, in vehicles and on foot. The aggressive pursuit was potentially dangerous and worrying for them," Harverson said.
The photos of the couple were published in the same week as pictures of William's mother, Princess Diana, leaving the Ritz Hotel in Paris in 1997, as part of the inquest into her death. The inquiry is charged with determining how Diana came to die with her boyfriend Dodi Fayed in a car crash while being pursued by paparazzi.
Harverson said it "seems incomprehensible, particularly at this time, that this behavior is still going on."
William and Middleton met at St. Andrews University in Scotland and dated for several years, but announced their split in April as speculation swirled that they were about to marry. Middleton had been pursued by photographers outside her London home and nightclubs in scenes that drew comparisons to the coverage of Diana.
Newspapers reported this summer that the pair had rekindled their romance.