The Duke of Sussex told the High Court he was unable to complain about the publisher of The Daily Mail "because of the institution I was in" as he gave evidence on the third day of the trial of his legal action.
In his witness statement for the trial, Harry said he has always had an "uneasy relationship" with the press, adding: "However, as a member of the institution the policy was to 'never complain, never explain'.
His appearance came as his father, the King, carried out engagements in London.
Prince Harry, 41, who was dressed smartly in a blue suit and a striped tie, briefly gave a thumbs up as he made his way into the central London court.
The Duke is part of a group of high-profile claimants, which include Sir Elton John and his husband David Furnish, campaigner Baroness Doreen Lawrence, politician Sir Simon Hughes, and actresses Sadie Frost and Liz Hurley. They are bringing legal action against Associated Newspapers Limited (ANL) over allegations of unlawful information gathering.
This includes claims that information for articles was obtained by carrying out or commissioning unlawful activities such as phone tapping and "blagging" private records. ANL has strongly denied wrongdoing and is defending the claims.
Harry also attended court on Monday and the start of proceedings on Tuesday, but was seen leaving the Royal Courts of Justice in London during the lunch break.
He was scheduled to give evidence on Thursday, but opening submissions for the claimants and ANL concluded earlier than expected on Tuesday.
ANL has strongly denied wrongdoing and is defending the claims.
Harry also attended court on Monday and the start of proceedings on Tuesday, but was seen leaving the Royal Courts of Justice in London during the lunch break.
He was scheduled to give evidence on Thursday, but opening submissions for the claimants and ANL concluded earlier than expected on Tuesday.
A source close to the Duke of Sussex's claim said: "ANL, publishers of the Daily Mail, Mail on Sunday and MailOnline, and their legal team, have had months to inform the court that their opening argument would last less than two hours, instead, they have had to resort to game playing and dirty tricks – consistent with the way they have treated not just the Duke but all of the victims in this case.”
The source continued: "They think that by pulling the schedule forward 24 hours they are giving Prince Harry less time to prepare – he's been preparing for this moment for the last three years. Safe to say, he's ready."
The trial is due to conclude in March, with a judgment due in writing at a later date.
Meanwhile, King Charles attended a reception in support of Indonesian conservation efforts at Lancaster House, next door to his Clarence House residence, on Wednesday morning. The monarch was joined by the president of Indonesia Prabowo Subianto, with the pair due to have an audience together afterwards at nearby St James's Palace.
Charles was away in Scotland at the start of the week as his youngest son flew in from California for the start of the trial. Harry is not expected to see his father during his UK visit, with the King following the general principle that a sovereign should steer clear of any active legal proceedings.
Harry's witness statement
The Duke told how he felt like his "every move, thought or feeling was being tracked and monitored" and how the alleged actions created "distrust and suspicion” and “driv(ing) me paranoid beyond belief, isolating me" in extracts of his witness statement provided on the first day of the trial on Monday.
Harry was "caused great distress by each and every episode of UIG (unlawful information gathering) against him by Associated or on its behalf, and the fruits of that UIG in the 14 unlawful articles of which he complains", his barrister David Sherborne said in written submissions.
The barrister said that the 14 articles involved in Harry's claim, written between 2001 and 2013 "focus primarily and in a highly intrusive and damaging way, on the relationships which he formed, or rather tried to form, during those years prior to meeting his now wife Meghan, the Duchess of Sussex".
ANL's response
In court on Tuesday, Antony White KC, for ANL, said the claims against the company were "threadbare" and had been brought too late.
He continued that its journalists provide a "compelling account of a pattern of legitimate sourcing" of the more than 50 articles that are alleged to be the products of unlawful information gathering.
In written submissions, Mr White said the publisher "strongly denies" that there was any unlawful information gathering, including voicemail interception, directed at the Duke.
He continued that the articles "were sourced entirely legitimately from information variously provided by contacts of the journalists responsible, including individuals in the Duke of Sussex's social circle, press officers and publicists, freelance journalists, photographers and prior reports".
Additional reporting by PA.