A private investigator working for Rupert Murdoch's News of the World conducted surveillance on Prince William as well of dozens of politicians and celebrities, the BBC reported Tuesday.
The broadcaster said private eye Derek Webb spied on the prince in 2006 while William was in Gloucestershire, western England, where his father Prince Charles has a country home.
Webb told the BBC that royal protection officers "had no idea" about the surveillance.
The newspaper's parent company, News International, said it was "not able to make any comment around the specific work carried out by Derek Webb."
On Monday the firm acknowledged that it had spied on two lawyers who represent alleged victims of phone hacking by the News of the World.
News International said the surveillance had not been illegal but was "deeply inappropriate."
William's office declined to comment on security matters. The tabloids have long had an intense interest in his private life, particularly during his bachelor years.
The BBC said Webb worked for the newspaper for eight years until it was shut down in July and was paid to follow more than 100 people, most of them celebrities and politicians.
It said the targets included Angelina Jolie, Prince Harry's former girlfriend Chelsy Davy, then-Attorney General Peter Goldsmith, former soccer star Gary Lineker and the parents of "Harry Potter" actor Daniel Radcliffe.
"Basically I would write down what they were wearing at the time, what car they were in, who they met, the location they met, the times _ the times were very important _ and I would keep that," Webb told the BBC.
That form of surveillance would not be illegal, but media intrusion has become a major issue in Britain since evidence emerged earlier this year of widespread illegal eavesdropping by the News of the World.
Murdoch shut down the 168-year-old newspaper in July after it became clear that the tabloid's reporters had eavesdropped on the mobile phone voice mail messages of a missing teenage girl who was later found murdered.
That touched off a storm of public criticism that shook Murdoch's media empire and sent tremors through Britain's political, police and media establishments.
Several senior executives of Murdoch's News Corp. have resigned over the scandal, including former News of the World editor Rebekah Brooks and ex-Wall Street Journal publisher Les Hinton.
More than a dozen journalists, most of them former News of the World employees, have been arrested and questioned about phone hacking, though none has yet been charged. Dozens of people _ from celebrities and politicians to the families of crime victims _ are suing Murdoch's News Corp.
Media scrutiny is particularly sensitive when it concerns the royal family. Many people still blame the intense press interest in William's mother Princess Diana for helping contribute to her death in a car crash in Paris in 1997.
Rupert Murdoch and his son James, who heads News Corp.'s international division, both denied knowing hacking was endemic at the newspaper when they appeared before a panel of British lawmakers investigating phone hacking in July.
Former News Corp. employees have cast doubt on their testimony, and James Murdoch has been recalled to give evidence to the lawmakers again on Thursday.