The leak of a mobile phone message about a planned interview with Prince William by ITN journalist Tom Bradby triggered the investigation which led to the arrest of News of the World royal correspondent Clive Goodman on Tuesday.
The police investigation is understood to have begun last November after details of a voicemail message left by Bradby, then ITN's royal correspondent, appeared in Goodman's News of the World diary column Blackadder.
Bradby said: "I was due to have a private meeting with William and I was pretty surprised to find that not only the details of the meeting, but what we were going to discuss pitched up in the News of the World the Sunday before [the meeting was due to take place].
"When he and I hooked up we both looked at each other and said, ‘How on earth did that get out?' We figured out that only he, I and two people incredibly close to him had actually known about it."
William revealed that it was not the first time that private information had appeared in the News of the World.
The prince said that details of a meeting he'd had with a knee surgeon — which only he, his personal secretary and the surgeon knew about — had appeared in the paper.
It was concluded that the information could only have been obtained by breaking into mobile answering machine messages, so a small-scale investigation was launched at Clarence House to deal with the incident.
The prince said that police working on the initial investigation uncovered information that alarmed them enough to hand it over to IT specialists from the anti-terrorism branch, who looked at it much more broadly.
He said the scope of the police investigation is understood to have widened to include celebrities and government ministers.