The phone-hacking trial was told Clive Goodman first hacked Kate Middleton’s voicemail in December 2005.
Clive Goodman also hacked Prince William 35 times and Prince Harry on nine occasions.
This is the first time the jury has heard of a royal’s phone being hacked.
Clive Goodman, who denies conspiring to commit misconduct in public office, previously said he only hacked aides.
He 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.
He returned to the Old Bailey to resume giving evidence after a long period away due to illness.
The court heard how Clive Goodman hacked Kate 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 August 7, 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.
This was the first time that the jury has heard that a mobile belonging to Prince William was hacked.
Clive Goodman 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.
The jury was also told that Clive Goodman hacked Michael Fawcett, a trusted aide of Prince Charles, 35 times.
Clive Goodman had not been in court since the end of March after he was declared unfit to carry on. The trial continued in his absence.
The judge told the jury he had been “ill”, but the court was given a report from an independent medical expert declaring him “now fit” to continue.
He also told the jury that he had “no alternative” but to keep them waiting because medical assessments were continually being submitted.
Clive Goodman will be allowed more time than usual to give the remainder of his evidence because medical experts have advised he may get tired more quickly.
The jury previously heard that the former royal editor of the now-defunct tabloid had undergone a minor heart procedure during the trial.
Clive Goodman, of Addlestone, Surrey, was convicted of phone hacking in 2006.
Appearing on Wednesday in the witness box, with a small bandage over his left hand, he was asked what he knew about phone hacking before January 2005, when he said he had been told about it by a colleague.
Before then, the News of the World colleague had passed pieces of information to Clive Goodman for stories. But the defendant said he never knew where it came from.
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