Prince Harry's years-long battle against Britain's tabloid media reaches its most dramatic stage so far this week, as the royal prepares to face questions in a London court as part of his case against a major newspaper publisher.

The latest hearing in the Duke of Sussex's case against Mirror Group Newspapers (MGN) began Monday at the High Court. Harry will give evidence in person Tuesday, his lawyer told the court.

The appearance of a British royal in a witness box will be an exceptionally rare event. But Harry has long railed against the tactics of the tabloid media in covering his life, and he will now have the opportunity to lay out his arguments, under oath, during cross-examination from MGN's lawyers.

It's likely to be a tense and defining appearance for the Duke as he forges his own path further from the rest of the royal family.

Here's what you need to know about the trial.

What is the case about?

The Duke of Sussex and three other claimants representing dozens of celebrities are suing MGN, accusing its titles of obtaining private information by phone hacking and through other illicit means, including private investigators, between 1991 and 2011.

The trial started on May 10, and is expected to last seven weeks.

MGN is contesting most of the allegations, arguing in its court filings that some claims have been brought too late and that in all four cases there is insufficient evidence of phone hacking.

In court documents published last month, the publisher did apologize for one instance of unlawful information gathering nearly 20 years ago. That incident involved a private investigator, who was paid £75 ($95) in 2004 by the Sunday People, a tabloid owned by the same group, to gather information about the Duke of Sussex while at a London nightclub.

Harry's lawyer David Sherborne has said his claim against MGN, which covers incidences from 1995 to 2011, is "significant not just in terms of time span but in the range of activity it covers."

Harry was subject to the most "intrusive methods of obtaining personal information," Sherborne said, arguing that "no one should be subjected to that." The "unlawful methods" were "habitual and widespread" among the journalists, Sherborne added.

What will Harry say?

When Harry enters the witness box on Tuesday he can expect a tough examination from the publisher's lawyers.

The prince alleges that about 140 articles published in titles belonging to the group contained information gathered using unlawful methods, and 33 of those articles have been selected to be considered at the trial, according to PA Media news agency.

It's likely that details of those stories will be parsed over at great length.

And while this is Harry's first appearance in a court case against the British media, it may not be his last.

The case against MGN is one of several lawsuits filed by Harry and his wife, Meghan, in their long-running battle with British tabloids, which they have accused of breaches of privacy and publishing false stories.

The pair has filed at least seven lawsuits against British and US media organizations since 2019, including Rupert Murdoch's News Group Newspapers (NGN), according to Reuters. NGN publishes the Sun and used to produce News of the World, which was shut down in 2011 over a phone hacking scandal.

In March, Harry appeared at a court hearing in his case against Associated Newspapers Limited (ANL) over allegations of unlawful information gathering, which that group has denied.

How will this impact the royals?

Harry's appearance in court is extremely rare. This is thought to be the first time a senior British royal will personally appear since 2002, when Princess Anne pleaded guilty after her dog bit two children in a Windsor park, according to PA Media.

It is more than 130 years since a senior member of the royal family gave evidence in court, when Edward VII was a witness in a slander trial over a card game in 1891, before he became king, Reuters reported.

It's not yet clear whether Harry's testimony will touch on other royals or his relationships with members of the family. But his brother and heir to the throne, Prince William, has recently been brought into the fray in another of Harry's cases.

Documents published in April as part of Harry's lawsuit against NGN allege that the publisher privately reached an undisclosed settlement with Prince William over historical phone hacking claims. It is unclear how Prince Harry is aware of his brother's settlement, but in his court response he writes that his information is based on a redacted document through which he is inferring that Prince William signed onto the agreement.

Officials at Kensington Palace, which represents Prince William, told CNN it does not comment on legal proceedings. Buckingham Palace reiterated that same position. Prince Harry states that his late grandmother, Queen Elizabeth II, was aware of settlement talks.

Whether this week's case will similarly bring questions for other members of the royal family remains to be seen. But it marks a watershed moment in Harry's efforts against major players in Britain's media, and his appearance is likely to dominate headlines in the days to come.