Prince Harry is one of those suing Associated Newspapers, the publisher of the Daily Mail, over alleged phone-tapping and other breaches of privacy.
The duke was seen at the High Court on Monday morning, while singer Sir Elton John, who is also involved in the legal proceedings, arrived at lunchtime.
The publisher has described the allegations as "preposterous smears".The duke, Sir Elton and actresses Sadie Frost and Liz Hurley are among the individuals who allege unlawful information gathering by the company, which also publishes the Mail on Sunday.
A four-day preliminary High Court hearing in London, starting on Monday, will consider legal arguments and a judge will decide whether the case will go any further. Associated Newspapers (ANL) is bidding to end the claims without trial.
Prince Harry's appearance will be seen by many as a sign of his strength of feeling over his privacy and determination regarding the legal action.