Former News of the World royal editor Clive Goodman hacked Kate Middleton's phone 155 times, a court has heard.
The phone-hacking trial was told Mr Goodman first hacked the now-Duchess of Cambridge's voicemail in December 2005.
He also hacked Prince William 35 times and Prince Harry on nine occasions, the BBC reports.
This is the first time the jury has heard of Price William's phone being hacked. Mr Goodman, who denies conspiring to commit misconduct in public office, previously said he only hacked aides.
Mr Goodman is one of seven defendants, including ex-News of the World editors Andy Coulson and Rebekah Brooks, on trial at the Old Bailey. They all deny the charges against them.
The court heard how Mr Goodman hacked Ms Middleton on Christmas Eve, Christmas Day and Boxing Day in 2005 - more than five years before she married Prince William.
He also hacked her on 7 August 2006, the day before he was arrested over allegations of phone hacking.
Prince William's voicemail was first intercepted in late January 2006, the court heard
Mr Goodman, 56, denied he had "forgotten" about targeting the young royals when he said in evidence earlier in the trial that he had only hacked aides working for the royals.
Upon his return to the witness box after an absence because of illness, he was accused of being more heavily involved in phone hacking than was previously heard.
Mr Goodman and private detective Glenn Mulcaire were convicted in 2006 of intercepting voicemails of royal aides. But jurors were told their activities went much further.
Under cross-examination by Mr Coulson's lawyer Timothy Langdale QC asked him why Ms Middleton had been targeted.
To which he replied Ms Middleton was a figure of increasing importance around the royal family.