Former royal editor Clive Goodman told the phone hacking trial that ex-News of the World editor Andy Coulson saw a transcript of a hacked voicemail left by Prince Harry.
Goodman told the Old Bailey that he was first alerted to the prince's message by private detective Glenn Mulcaire in December 2005 and then listened to it himself and made a transcript, which he went on to show Coulson, redacting the names with X and Y.
In an email read out in court, he told Coulson: "As you know it's 100% fact."
Asked by his lawyer lawyer, David Spens QC, why he says that, Goodman told the court it was because he had seen the transcript.
Goodman, 56, of Surrey, admitted phone hacking and was sentenced with Mulcaire in 2007.
He denies conspiring to commit misconduct in a public office. All the defendants in the case deny the charges against them.