Prince Harry comforted Liz Hurley's son, Damien, as the actress became visibly emotional in the witness box, crying as she was shown some of the articles relating to her claim.
The Duke of Sussex attended the fourth day of proceedings not to give further testimony himself, but to show support as the trial progressed beyond his own evidence.
Miss Hurley, 60, is suing over 15 articles which she alleges featured information obtained unlawfully, including details about her pregnancy with her son and arguments with his late father, Steve Bing.
Mr Hurley, now 23, watched from the back of the courtroom during his mother's questioning over the articles.
The prince comforted Miss Hurley as she became visibly emotional.
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The actress wept while she gave evidence, as Harry supported her son, placing his hand on the younger man's back.
Miss Hurley rejected any suggestion that her friend gave information to the press, having previously sued Mirror Group Newspapers for phone hacking and said she was paid £350,000 in damages, which she gave to pressure group Hacked Off.
Joining the Duke of Sussex and Miss Hurley in the legal action are Sir Elton John and his husband David Furnish, actress Sadie Frost, campaigner Baroness Doreen Lawrence, and former politician Sir Simon Hughes.
The claimants accuse Associated Newspapers Limited (ANL) of improperly obtaining private information about them.
Miss Hurley is suing over 15 articles, alleging they featured unlawfully obtained information.
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ANL has firmly rejected all allegations of wrongdoing and is mounting a robust defence against the claims.
The group represents a diverse coalition of public figures from entertainment, politics and activism, united in their pursuit of accountability from the newspaper group.
On Wednesday, the duke spent approximately two hours facing questions from ANL's legal team in what were described as frosty exchanges, before his own barrister, David Sherborne, asked him about his feelings regarding the proceedings.
Following his third day in court, Prince Harry declared: "Today we reminded the Mail Group who is on trial and why."
Harry comforted Mr Hurley while his mother testified.
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A spokesman for the duke said: "Today's cross-examination was revealing in its weakness: assertive in tone, but collapsing immediately under scrutiny from Prince Harry.
"Associated couldn't wait to get him off the stand, questioning him for just two hours and avoiding 10 of his 14 articles entirely."
ANL has firmly rejected all accusations of wrongdoing and continues to contest the claims.
Proceedings are scheduled to continue until March, when the trial is expected to conclude, with a judgment to be delivered in written form at a subsequent date.
Miss Hurley is one of seven claimants taking legal action against ANL.
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Passages from Prince Harry's Spare were read aloud in court in front of the Duke of Sussex during his evidence,
The prince was shown a historic newspaper article reporting a private conversation involving his former girlfriend, Chelsy Davy.
As questioning continued, the court was then referred to passages from Harry's memoir Spare in which he described the same period and events.
Extracts from the book were read aloud in court and put to the prince as part of the evidence being examined, according to the Times.
The references formed part of broader questioning about whether information contained in press coverage could have been obtained through lawful means.
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