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Clive Goodman, who was jailed in 2007 for illegally accessing voicemails on the mobile phones of royal aides, said he had hacked Queen Elizabeth's grandsons almost a decade ago in search of stories while working at the now-defunct tabloid.
From late 2005 until his arrest the following year, Middleton's phone was hacked 155 times despite her often changing the PIN number to access her voicemails, William's was hacked 35 and Harry's nine times, the court was told.
Goodman targeted Middleton, known as the Duchess of Cambridge since her marriage to William three years ago, on Christmas Eve, Christmas Day and Boxing Day in 2005. The last time was on Aug. 7, 2006, the day before police arrested Goodman.
Despite the regular royal hackings, Goodman said detectives had never before asked him about the tapping of the princes' phones and it had not been publicly disclosed before the trial.
The Old Bailey jury has previously heard how recordings of the royals' messages had been discovered, including one in which William called Kate "Babykins", but not who was responsible.
"I'm quite happy to get everything out there and in the open," Goodman told the court. "I don't want anyone to think I'm not ashamed of what we did."
Goodman and private detective Glenn Mulcaire, who worked for the paper, were jailed in January 2007, although the ex-royal editor's involvement was then said to have been limited to three aides to the princes.
At the time, Murdoch's British newspaper arm News International said phone-hacking was limited to a "rogue reporter" but police reopened their investigation when new information emerged in 2011.
The subsequent scandal rocked the British establishment and led Murdoch to close the 168-year-old News of the World. Three of the paper's former news editors have admitted phone-hacking offences.
Goodman, 56, is now on trial accused with the paper's former editor Andy Coulson, later Prime Minister David Cameron's media chief, of authorising illegal payments to police officers to obtain royal telephone directories.
Coulson and Rebekah Brooks, another former editor and later chief executive of Murdoch's British newspaper arm are also on trial accused of phone-hacking offences. They and four other defendants on trial deny the charges.
Goodman had been absent from court since falling ill in March part-way through cross-examination from Coulson's lawyer Timothy Langdale.
In earlier testimony, he said Coulson had agreed a project to fund a private detective to hack the phones of staff working for William and Harry.
Asked why he was now admitting hacking the royals themselves, Goodman said he could speak freely as the Crown Prosecution Service (CPS) had made it clear he would not be facing any further action over hacking.
"I'm happy to give a full account of every single one of these (hackings)," Goodman told the court. He said he had also hacked the phones of a personal assistant to Sarah Ferguson, the Duchess of York, and Michael Fawcett, an aide to heir-to-the-throne Prince Charles.
Langdale queried why he had not disclosed these details in response to questions put to him weeks ago, when he only recalled hacking five royal aides and the son of Camilla Parker Bowles, the second wife of Prince Charles.
"Had you really forgotten that you had been hacking him (Prince William)?" Langdale asked.
"I didn't recall specifics," Goodman replied. "I have not been asked it before."
The trial continues.