A private investigator carried out surveillance of Prince William and scores of other targets for the News of the World, it has been claimed.
Derek Webb was paid to follow and record the movements of celebrities picked by the newspaper's staff, the BBC reported.
The investigator told Newsnight: "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."
He continued: "And then I would transfer part of it into my diary, but not the actual log itself. Just the names of the people."
A spokesman for the Duke of Cambridge declined to comment.
Mr Webb told the broadcaster that over eight years he was paid to follow more than 90 targets including former attorney general Lord Goldsmith and football pundit Gary Lineker.
Relatives, such as the parents of actor Daniel Radcliffe, were also targeted, he said.
In 2006 Mr Webb was asked to follow the Prince when he was spending a number of days in Gloucestershire, it was claimed.
The investigator, a former policeman, told Newsnight: "I was working for them extensively on many jobs throughout that time."
Carrying out surveillance is not illegal and is not new for journalists or private investigators.
