Prince Harry is back in the UK for his court case (Picture: REUTERS)

Prince Harry will face hours of questioning at the High Court today in his case against Mirror Group Newspapers (MGN) – but what is the lawsuit all about?

He will become the first royal to testify in court for 130 years in a lawsuit – the last to do so was Edward VII, who appeared in 1870 as a witness as part of a divorce case and 20 years later in a slander trial over a card game, both before he became king.

In 2002, Princess Anne appeared in court to plead guilty after her dogs bit two children.

Today, the Duke of Sussex, 38, will be questioned under oath on topics such as his relationship with ex-girlfriend Chelsy Davy, and is expected to face quite a grilling.

He is also expected to face questions about his dealings with former Mirror editor Piers Morgan.

So what is the trial about and why is the Royal appearing in person?

Here’s what you need to know.

Why is Prince Harry in court?

Prince Harry is part of a case involving himself and more than 100 other celebrities against Mirror Group Newspapers (MGN) – publishers of The Daily Mirror, Sunday Mirror and Sunday People, now owned by Reach Plc.

The Duke’s relationship with ex girlfriend Chelsy Davy will be under scrutiny (Picture: Max Mumby/Indigo/Getty Images)

The trial started last month and involves allegations that MGN journalists unlawfully gathered information, which Harry’s lawyers claim was carried out with the knowledge and approval of senior editors and executives.

They claim 140 articles published between 1996 and 2010 used unlawfully gathered information.

MGN said it ‘unreservedly apologises’ to the duke for one incident when a journalist unlawfully sought information about one of Harry’s nights out in 2004 at London’s Chinawhite nightclub – a known celebrity haunt which has since closed – and that he was entitled to ‘appropriate compensation’.

It has previously admitted its titles were involved in illegally gathering information and has settled more than 600 claims at a cost of more than £100 million pounds in damages and costs. But its lawyers said there was nothing to suggest any other unlawful activity relating to the royal.

Which other famous faces are involved in the case?

The Duke’s case is being heard alongside three other claims by representatives of Coronation Street actor Michael Turner – known professionally as Michael Le Vell – former Coronation Street actress Nikki Sanderson, and comedian Paul Whitehouse’s ex-wife Fiona Wightman.

Prince Harry’s barrister was ‘roasted’ in court yesterday after the Royal didn’t appear (Picture: PA/Reuters)

Harry was expected to appear in court yesterday but failed to show up, leaving his barrister to take a ‘roasting’. The judge, Timothy Fancourt, said he was ‘surprised’ by Harry’s absence while MGN’s lawyers described it as ‘absolutely extraordinary’.

What is Harry claiming?

The Duke’s lawyers claim his privacy was ‘invaded by these three newspapers using unlawful methods and it is the use of these methods by a national media group that has brought him here, not some vendetta against the press generally’.

Outlining his case, his lawyer David Sherborne said some 2,500 articles had appeared about Harry’s private life in the MGN titles during the period, which also covers the death of his mother, Princess Diana.

He suggested the late Princess’s phone had been hacked, and said an article from 2003 about a disagreement with Prince William showed ‘the seeds of discord between these two brothers are starting to be sown’.

Harry’s lawyers also claim then tabloid editor Piers Morgan had ‘clear involvement [in] and knowledge’ of the alleged illegal activities, which Morgan denies.

Today he could be questioned for several hours about his relationships, his mother and life in the Royal Family – while the BBC reports that his memoir, Spare, might be turned against him, with its accounts of drug taking and family tensions.

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Find out what the case is all about.