Other prominent figures who are suing include Sir Elton John, David Furnish, Elizabeth Hurley, Sadie Frost and Doreen Lawrence.
David Sherborne, the lawyer representing Harry has said, "Prince or not, the blatantly unlawful and illegal methods that were used by the defendant . was quite frankly appalling.
"No-one should have been subjected to that."
WATCH: Prince Harry arrives at High Court in London. Article continues after video.
Previously, Prince Harry made the allegation in court documents that his brother, Prince William was also preparing to fight the publisher. Rather than appearing in court, the Duke of Sussex alleged that his brother settled for "a very large sum of money".
The trial is set to continue for seven weeks, in which Harry will appear in person in June.