Piers Morgan, former editor of the Daily Mirror, is accused of unlawfully obtaining private information about Prince Harry and injecting it into stories written by Jane Kerr, a former royal editor for the Daily Mirror.
Kerr testified that Morgan would add information to her stories without explaining where it came from, and that he had a "genuine interest" in the coverage.
Prince Harry's lawyer questioned Kerr about a story reporting that Prince Charles had been "hugely relieved" to have been told his son had only used cannabis "spliffs," which Kerr said she could not remember where she got the information from.
Kerr also wrote a story about Prince Harry catching glandular fever, and said she could have got the information from the Palace, though the media had been told that Prince Harry and Prince William
were "off-limits" to reporters at the time.
The trial is examining whether information in 140 articles published by Mirror Group Newspapers (MGN) between 1996 and 2010 was obtained illegally.
Phone hacking occurred on an "industrial scale" at MGN papers.
Illegal methods were used to gather information on Prince Abdul Aziz, including private matters.
Other claimants involved in the case, including Coronation Street actors, allege that unlawful methods were used to obtain information for stories.
They claim that senior executives knew about it and failed to stop it.
MGN denies these allegations.
The publisher argues that some of the claimants brought their legal action too late.