LONDON (Reuters) -Prince Harry will become the first British royal to appear in the witness box since the 1890s when he testifies this week at the High Court in London as part of his lawsuit against Mirror Group Newspapers.

Here are details of the trial:

WHAT IS THE COURT CASE ABOUT?

Harry and more than 100 other people are suing Mirror Group Newspapers (MGN), publisher of the Daily Mirror, the Sunday Mirror and the Sunday People tabloids, accusing them of widespread unlawful activities between 1991 and 2011.

Those involved include actors, sports stars, celebrities and people who simply had a connection to high-profile figures.

They say the media group's journalists or private investigators commissioned by them carried out phone-hacking on an "industrial scale", obtained private details by deception and carried out other illicit acts to uncover information about them.

Senior editors and executives knew and approved of the behaviour, the claimants' lawyers say.

MGN is contesting the claims and denies senior figures were aware of wrongdoing. It also argues some of the lawsuits were brought too late.

Harry, the younger son of King Charles, was selected at an earlier hearing as one of four test cases for the trial which began on May 10.

He is due to give evidence when his specific case is heard over three days, starting on Monday.

WHAT IS PHONE HACKING?

Phone hacking, the illegal interception of voicemails on mobile phones, first came to attention in 2006 when the then- royal editor of Rupert Murdoch's News of the World (NoW) tabloid and a private investigator were arrested.

They pleaded guilty and were jailed in 2007. The NoW and senior figures at Murdoch's News Group Newspapers (NGN) UK operation said the hacking was limited to a rogue reporter.

But further revelations in 2011, including that a murdered schoolgirl had been targeted, led to the closure of the paper and a criminal trial.

In 2014, the NoW's former editor, Andy Coulson, who later worked for ex-Prime Minister David Cameron, was found guilty of conspiracy to hack phones and jailed. Rebekah Brookes, who heads up Murdoch's UK newspaper and radio operation, was acquitted of all charges.

The Mirror group had consistently denied its journalists had been involved in hacking, including at a public inquiry, but in 2014 it admitted liability in four cases.

Since then, MGN has settled more than 600 claims at a cost of over 100 million pounds ($120 million) in damages and costs.

WHAT DOES HARRY SAY THE MIRROR GROUP NEWSPAPERS DID?

Harry says 140 stories which appeared in MGN papers were the result of phone-hacking or other unlawful behaviour; however, the trial is only considering 33 of these.

His lawyers said the intrusion led to the breakdown of his relationship with a long-term girlfriend, Chelsy Davy.

In other documents released at the outset of the trial, MGN admitted there was evidence that private investigators had been instructed to unlawfully gather information about three of those involved in the test cases, including, on one occasion, Harry.

The publisher said it unreservedly apologised and that he was entitled to compensation.

However it has rejected his other claims. In documents, it says some of the information had been passed on by a former senior aide working for his father.

WHY IS HARRY TAKING ACTION?

The MGN case is one of four that Harry is currently pursuing at the High Court against British newspapers.

He is also suing Murdoch's NGN, which publishes the Sun tabloid and used to produce the defunct NoW, over alleged phone-hacking and other unlawful acts. NGN denies the Sun was involved in wrongdoing and is fighting to have his case thrown out.

The prince, with singer Elton John and five others, is also suing Associated Newspapers (ANL), publisher of the Daily Mail and Mail on Sunday, over phone-hacking and illicit privacy breaches. ANL denies any unlawful activity.

Harry is also suing ANL for libel.

"This isn't just about phone hacking, this is about accountability of power," he wrote in a statement as part of the NGN case. He says the press was too important to have "criminals masquerading as journalists running the show".

(Reporting by Michael Holden; Editing by Janet Lawrence and Bernadette Baum)