The apparent leak of information from a conversation between ITN journalist Tom Bradby and the Prince of Wales' press office to the News of the World triggered the investigation which led to the arrest of the paper's royal correspondent yesterday.
Clive Goodman is currently still being detained by police under the Regulation of Investigative Powers Act – after he was arrested last night – and they have since gained a 12-hour extension.
The police investigation is understood to have begun a year ago after details of a conversation that Tom Bradby, then ITN's royal correspondent, had with Clarence House appeared in a News of the World diary story shortly after.
This raised suspicions as to how the information, which only a small number of people within Clarence House would have had access to, was leaked.
An ITN source told Press Gazette: "Tom was talking to Clarence House about a potential ITV news project involving Prince William. The conversations that were had about that were between a very small number of people. Shortly thereafter, details of that appeared in an News of the World diary piece."
The investigation started on a small scale, looking into the matter of the leak involving Bradby.
The source said: "On the back of that the Palace contacted the police who then launched an investigation into that as obviously it is a security issue."
However its scope increased is since undetrstood to have increased as celebrities, government ministers and the MOD were possibly implicated as well.