The News of the World's former royal editor Clive Goodman has told the hacking trial that phone hacking was going on at the paper "on a pretty industrial scale".
Goodman named another senior journalist at the NoW who believed he had an "exclusive" deal with private investigator Glenn Mulcaire.
He told the court he kept his own arrangement secret from his colleague, who cannot be named for legal reasons.
His own project with Mulcaire was only due to last a couple of months from October 2005 but went on ad hoc until the time of their arrests in August 2006, he said.
Goodman gave some examples of stories he got – one about Prince William getting drunk at his brother's passing out parade, and another about Prince Harry asking his private secretary for help with his homework at Sandhurst.
Asked if he ever told the senior journalist where he got hacking stories from, he said: "He may well have suspected but we never had a conversation like that.
"The problem was Glenn Mulcaire was supposed to be working exclusively for (senior journalist). It would have caused conflict."
He denied telling co-defendant Andy Coulson that he had got a story from hacking.
Asked about the extent of phone hacking, Goodman said: "It got to the level when (the senior journalist) was hacking the editor Andy Coulson's phone."
Asked for what purpose, he said it was to find out what other stories were in the pipeline in the competitive newsroom.
He was also hacking Rebekah Brooks' phone while she was editor on the Sun "for the same reason – to find out what the Sun was up to", Goodman said.
The hacking trial was told today that a phone message from Prince William to Kate Middleton was intercepted by a private investigator working for the News of the World.
The message recorded by Glenn Mulcaire was of William arranging to leave Sandhurst to meet his then girlfriend, the Old Bailey heard.
It was found on a micro-cassette in former royal editor Clive Goodman's flat.
Goodman, 56, was asked about the recording during his third day in the witness box.
His lawyer, David Spens QC, read out part of a police transcript on the tape dated February 23 2006.
The judge, Mr Justice Saunders, asked him to only read the relevant parts and not the whole message.
In it, William says: "Hopefully I should be able to leave by seven at the latest."
Later in the message he repeats: "Planning on definitely coming out by sevenish so I will be with you by eight at the latest."
Spens asked Goodman: "How did this voice message from Prince William to Kate Middleton come to be on a micro-cassette?"
Goodman replied: "This was originally recorded by Glenn Mulcaire."
Goodman admitted phone hacking and was sentenced in 2007. He denies conspiring to commit misconduct in a public office.
Goodman told the court that he made a personal deal with Mulcaire to pay him £500 a week to get information for hacking the phones of people close to princes William and Harry, separate from the newspaper's contract.
He gave the private investigator the names of the princes' personal assistant, private secretary and also the Prince of Wales's head of press.
Spens said: "Was there any advantage to you in giving him the names of these three people?"
He replied: "I would be able to access information about meetings, events, events happening in the lives of the Prince of Wales's sons, who were then up and coming and making their way in the world."
All the defendants in the case deny the charges against them.
Goodman told the court that he got the story about Prince Harry's homework from his private secretary's voicemail.
He was first alerted by Mulcaire in December 2005, then listened to it himself and made a transcript, which he went on to show Coulson, redacting the names with X and Y.
In it, the Prince asks for information about the Iranian Embassy siege, saying, "Please, please, please, email it to me or text me and I will try to ring you again later."
Goodman discussed his progress on the story in emails with Coulson and the senior journalist, the court heard.
He told the senior journalist that Coulson had a "full briefing last night and his verdict was 'Steam in'".
He later emailed the News of the World editor that he was going to contact the Prince's private secretary, Clarence House and the Ministry of Defence to stand up the story.
In the email read out in court, he told Coulson: "As you know it's 100% fact."
Asked by Spens why he said that, Goodman told the court it was because he had seen the transcript.
In an email to Coulson, he went on to say that a reference to the particular subject Harry wanted help with – the Iranian Embassy siege – was "too precise to get through unnoticed so I'm going to include it in a list of topics cadets would be asked about".
Asked why he did that, he said the detail would be "fairly transparent".
The court heard of more stories about the young royals which came from hacking, including one about Prince William getting lost in woods during a night exercise and getting shot with blanks, much to his embarrassment.
Coulson had agreed Goodman's monitoring deal with Mulcaire dubbed Project Alexander, the jury was told.
The private detective was referred to in emails between them as Matey, the court heard.
In an email to Coulson in February 2006, Goodman described how cost-effective the project was for producing big stories.
He told Coulson: "A few weeks ago you asked me to find new ways of getting into the family, especially William and Harry, and I came up with this safe, productive and cost-effective way.
"I'm confident it will become a big story goldmine for us if you just let it run just a little longer."