Prince Harry has brought a lawsuit against Associated Newspapers (ANL), along with John, and his husband David Furnish, as well as actors Elizabeth Hurley and Sadie Frost.
The Duke of Sussex and singer John made a surprise appearance at London's High Court on Monday to attend the hearing against a UK's publisher over years of alleged phone-tapping and privacy breaches.
The group's lawyer David Sherborne said his clients were victims of "numerous unlawful acts" carried out by journalists or private investigators working on behalf of the papers.
King Charles younger son defended his brother Prince William and sister-in-law Kate Middleton's mom in his legal battle as he accused the papers of using unlawful means to obtain stories about him from at least early 2001 until at least late 2013, saying his brother William and the mother of Kate had also been targeted.
The document alleges they had sought information about private flight details of his ex-girlfriend Chelsy Davy, and hacked mobile phone voicemail messages and bugged the landlines of his friends, with a hardwire tap put on the phone on one of them, Guy Pelly.
The actions had meant he was "largely deprived of important aspects of his teenage years" and the suspicion and paranoia they caused led to him losing friends as "everyone became a 'suspect'", the claim said.
The unlawful attempts to find out details of the royal's private travel plans were also a significant security risk "which was as grossly irresponsible as it was dangerous", the document said.
In court submissions, ANL said the claims were based on inference rather than evidence, and that the claimants had provided little or no evidence of unlawful information gathering by its journalists – which it strongly denies.