The long-awaited case alleges that the publisher of the Mirror illegally gathered information on the Duke of Sussex and a number of other celebrities to generate stories.

What is this case about?

The High Court is hearing a seven-week long trial into allegations from Prince Harry and others that their phones were hacked by journalists from Mirror Group Newspapers (MGN).

The Duke of Sussex will give evidence in person in June - meaning he will be the first senior member of the Royal Family in modern times to appear in court and be cross-examined.

What do the claimants mean when they allege phone hacking?

The allegations concern stories dating back, in some instances, more than 20 years.

The claimants [those who believe their phones were hacked] allege that journalists from the Daily Mirror, Sunday Mirror and The People newspapers obtained private and confidential information about their lives through a variety of illegal means.

They believe that journalists exploited a (now-closed) gap in the security of mobile phones which allowed them to access the voicemail of their targets. By listening to messages left by friends and family, they were able to find out about the lives of the people they wanted to write about.

Journalists also allegedly used private investigators to trick others into handing over confidential information about the targets - everything from financial details to medical information.

For example, Prince Harry says there is evidence that reporters and photographers used a variety of techniques to follow the movements of his former girlfriend Chelsy Davy.

The claimants say there was no justifiable public interest in their targeting - it was simply "wrongdoing for cynical commercial reasons".

Aside from Prince Harry, who else is involved in the case?

Alongside the Duke of Sussex, allegations from three other claimants are being tested: Coronation Street actor Michael Le Vell (real name Michael Turner), Nikki Sanderson; and Fiona Wightman, the ex-wife of the comedian Paul Whitehouse.

The four cases were chosen by the trial judge to help the court set the level of damages Mirror Group Newspapers should pay if the claimants win.

The High Court would then consider other cases from celebrities including the former Girls Aloud singer Cheryl, the estate of George Michael, actor Ricky Tomlinson and former Arsenal and England footballer Ian Wright

What's the evidence ?

At the heart of the trial will be 207 newspaper stories published between 1991 and 2011.

Some 140 of them, dating from 1996 to 2010, were about Prince Harry.

Former Daily Mirror editor Piers Morgan has repeatedly denied involvement in illegal phone hacking

The claimants argue senior executives must have known about unlawful information gathering behind these stories and failed to stop it.

One of the key allegations in the case is that the TV presenter Piers Morgan, editor of the Daily Mirror between 1995 and 2004, knew of illegal activity.

The Duke of Sussex's lawyers want the court to consider a series of incidents which they say are evidence that Mr Morgan not only knew about hacking but told others too.

Mr Morgan has repeatedly denied involvement in illegal phone hacking - but this is the first time that a court will have been asked to rule on allegations about what he knew.

What has MGN said?

Mirror Group Newspapers has previously admitted that phone hacking took place.

In 2015, following allegations made by former Coronation Street actress Shobna Gulati, Mirror Group Newspapers admitted journalists had regularly used unlawful techniques and paid investigators to obtain private information. It issued a public apology.

Mirror Group Newspapers has settled hundreds of claims - the 2015 case saw it pay out £1.25m in damages to eight victims. MGN has set aside £28m to deal with hacking allegations.

However, the company insists the blame cannot be pinned on executives because it says the unlawful activity was deliberately concealed by the journalists.

Weren't hacking allegations settled years ago?

Yes and no. There have been more than 1,000 claims against the owners of The Sun and long-closed News of the World (NOTW), owned by the Rupert Murdoch empire. Some NOTW journalists were jailed in relation to unlawful intrusion.

In 2012, the separate Leveson Inquiry revealed widespread allegations of wrongdoing in the tabloid press - but that investigation was not set up to deal with damages - and so a huge number of cases have rumbled on for more than a decade.

In the coming weeks, the claimants will allege that Mirror Group misled the Leveson Inquiry - something it denies.

Who else is Prince Harry taking action against?

The duke is suing News Group Newspapers (NGN), publisher of The Sun for alleged hacking. Its lawyers are trying to have the case thrown out by arguing that Prince Harry has run out of time to bring it to court. The paper's owners have long admitted hacking took place at the News of the World - but never conceded it happened at The Sun.

Separately, the duke is one of seven people who allege unlawful intrusion by the Daily Mail and Mail on Sunday. The newspaper group vehemently denies that allegation.

The future of these cases will be decided later this year.