The Duke of Sussex is not suing the Daily Mirror’s publisher because of a “vendetta against the press” but to hold them to account, the High Court heard on the eve of Harry’s appearance in the witness box.

On Monday, the duke’s individual case against Mirror Group Newspapers (MGN) over alleged unlawful information gathering got under way.

Barrister David Sherborne claimed the duke was subjected to unlawful information-gathering activity “right from when he was a young boy at school” into adulthood, adding: “Nothing was sacrosanct or out of bounds.”

He continued: “Every part of the prince’s life during these years was invaded by these three papers using these unlawful methods.”

The barrister said, contrary to some criticism of Harry, it was “the use of these methods by a national media group that has brought him here, not some vendetta against the press generally”.

Harry is suing the publisher, claiming journalists at its titles – which also include the Sunday Mirror and Sunday People – were linked to methods including phone hacking, so-called “blagging” or gaining information by deception, and use of private investigators for unlawful activities.

MGN is contesting the claims and has either denied or not admitted each of them. The publisher also argues that some of the claimants have brought their legal action too late.

Mr Sherborne has previously claimed that unlawful information gathering was “habitual and widespread” at MGN’s titles over almost two decades.

The barrister told the court on Monday that Harry, 38, was “the biggest target”, adding that it was “implausible” there were no admissions from MGN except one.

He claimed that Harry’s phone “would have been hacked on multiple occasions”, adding that his details appeared in the PalmPilot of a journalist who was one of the “most prolific” phone hackers.

Mr Sherborne later said: “What he has been able to do as a result by bringing this claim, he has been able to focus the attention that comes with his position on these activities and the fact that they have been carried out not just by journalists, but they have been concealed, even though they were known about by the senior members of the board of this PLC, as well as the legal department”.

“It is the focus on these activities and bringing the defendant to account which is why he is bringing this action.”

The duke had been expected to arrive at court on Monday but will not appear until Tuesday.

The hearing in London heard Harry had flown to the UK from Los Angeles in the US last night, as he was celebrating his daughter Lilibet’s second birthday on Sunday.

Brothers can sometimes disagree, but once it is made public in this way and their inside feelings revealed in the way that they are, trust begins to be eroded

Mr Justice Fancourt, the judge hearing the case, said he was “a little surprised” to hear that Harry would not be attending court on Monday.

Set to enter the witness box on Tuesday morning, Harry is due to face up to a day-and-a-half of cross-examination from MGN’s barrister.

It is thought to be the first time a senior member of the royal family has personally appeared in court proceedings since 2002, when the Princess Royal pleaded guilty to a charge under the Dangerous Dogs Act after her pet bit two children in Windsor Great Park.

Harry alleges about 140 articles published between 1996 and 2010 contained information gathered using unlawful methods, and 33 of these have been selected to be considered at the trial.

Mr Sherborne told the High Court in London that “no aspect of the young prince’s life was safe” from press intrusion and that details of his relationship with ex-girlfriend Chelsy Davy “were all revealed and picked apart by the three Mirror Group titles”.

The court was later told that a 2003 article detailed an alleged row between the duke and his brother William, now the Prince of Wales, over their mother’s former butler, Paul Burrell.

“Brothers can sometimes disagree,” Mr Sherborne continued, adding: “But once it is made public in this way and their inside feelings revealed in the way that they are, trust begins to be eroded.”

Mr Sherborne also said the late Diana, Princess of Wales, was a “huge target” for MGN’s newspapers, adding that certain unlawful activities in relation to her would have also affected Harry.

He read out two letters from Diana to entertainer Michael Barrymore, which referred to private meetings between the pair, and in one of the letters Diana referred to being “devastated” to learn that the “Daily Mirror” had contacted her office about him and their meetings.

However, Andrew Green KC, for MGN, said Harry’s claim had “become rather fantastical”.

The barrister said: “The defendant’s position is that there is simply no evidence capable of supporting the finding that the Duke of Sussex was hacked, let alone on a habitual basis.”

He said that payment records used in the duke’s claim “simply do not demonstrate unlawful conduct or knowledge thereof”.

He also said that there was a lack of call data in Harry’s case, telling the court: “There is no call data whatsoever for the Duke of Sussex and scant call data for his associates.”

Mr Green later said the duke faced “a very difficult starting point for the claimant proving he was habitually hacked”.

The barrister also said a suggestion that its journalists had hacked the phone of Diana, the late Princess of Wales, was “total speculation”.

He told the court: “Mr Sherborne’s suggestion that MGN’s journalists hacked her phone is total speculation without any evidential basis whatsoever”.

“The letters you were shown, to Michael Barrymore, are not evidence of voicemail interception,” Mr Green said, adding that “plainly no such finding could be made”.

Harry’s claim is being heard alongside three other “representative” claims during a trial which began last month and is due to last six to seven weeks.

The three other representative claimants are Coronation Street actor Michael Turner, known professionally as Michael Le Vell, who is best known for playing Kevin Webster in the long-running soap, former Coronation Street actress Nikki Sanderson, and comedian Paul Whitehouse’s ex-wife Fiona Wightman.

Mr Green said voicemail interception was denied in all four cases and that there was “no evidence or no sufficient evidence”.

The barrister continued: “There is some evidence of the instruction of third parties to engage in other types of unlawful information gathering in respect of each of the claimants, save for Mr Turner whose claim is entirely denied, and MGN has made pleaded admissions in respect thereof.

“MGN unreservedly apologises for all such instances of unlawful information gathering, and assures the claimants that such conduct will never be repeated.”