The publisher has described the allegations as "preposterous smears".

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. ANL is bidding to end the claims.

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.

Others taking part in the legal action include Sir Elton's husband David Furnish, and Baroness Doreen Lawrence, the mother of Stephen Lawrence, who was murdered in a racist attack in 1993.

The group launched the legal action last year after becoming aware of "compelling and highly distressing evidence that they have been the victims of abhorrent criminal activity and gross breaches of privacy" by ANL, according to a statement by law firm Hamlins released in October 2022.

Court proceedings began with a bid by ANL's lawyers to have certain reporting restrictions imposed in the case.

The Duke of Sussex sat towards the back of the courtroom, occasionally taking notes in a small black notebook as legal arguments were made by ANL's barrister Catrin Evans KC.

Frost sat two seats away from Harry in the courtroom.

When the legal action was announced in October, law firm Hamlins, which is representing Prince Harry and Frost, alleged that ANL's activity included:

"The hiring of private investigators to secretly place listening devices inside people's cars and homes

"The commissioning of individuals to surreptitiously listen into and record people's live, private telephone calls whilst they were taking place

"The payment of police officials, with corrupt links to private investigators, for inside, sensitive information

"The impersonation of individuals to obtain medical information from private hospitals, clinics, and treatment centres by deception

"The accessing of bank accounts, credit histories and financial transactions through illicit means and manipulation"

In a statement released at the time, the newspaper group said: "We utterly and unambiguously refute these preposterous smears which appear to be nothing more than a pre-planned and orchestrated attempt to drag the Mail titles into the phone hacking scandal concerning articles up to 30 years old.

"These unsubstantiated and highly defamatory claims - based on no credible evidence - appear to be simply a fishing expedition by claimants and their lawyers, some of whom have already pursued cases elsewhere."

The duke's appearance on Monday is believed to be the first time he has been back in the UK since the late Queen's funeral in September.

His surprise return comes nearly three months after he publicised his troubled relationship with his father the King and brother the Prince of Wales in his controversial autobiography Spare.

The Duke and Duchess of Sussex also released a Netflix documentary in December, titled Harry & Meghan.

The King was due to be away on Monday on the first official state visit of his reign, but the trip to France was cancelled due to rioting over pension reforms.

He is due to leave for a state visit to Germany on Wednesday morning. Buckingham Palace said he was not in Windsor or London on Monday.

The Duke and Duchess of Sussex have been asked to vacate their UK home, Frogmore Cottage on the Windsor estate, in a move sanctioned by the King.

The duke is also taking legal action against the Home Office over security arrangements when he is in the UK, raising questions about his own security provisions during this visit.