The royals aren't public property – and neither are their private exchanges with loved ones

In ruling that being a royal does not make "one" public property, Mr Justice Warby has undoubtedly chalked up a significant gain not only for the Duke and Duchess of Sussex but also for the Royal family as a whole.

Members of the House of Windsor have long argued that they reserve the right to a private life despite having to endure more intrusion than the rest of us.

Few have ever sued, however, preferring to follow the Queen's tried and tested mantra – "never complain, never explain" – for fear of invading their privacy even further by taking cases to court. Even when they have thrown down the gauntlet, they have sought to avoid giving evidence in person, preferring instead to rely on sworn affidavits by palace aides.

Few will forget how the 2002 trial of the former Royal butler Paul Burrell for allegedly stealing items belonging to the late Diana, Princess of Wales, was dramatically halted when the Queen had a "recollection" that he had told her he was keeping some of her possessions. It meant no one royal ended up in the witness box.

When Prince Charles sued the Mail on Sunday in 2005 for publishing extracts from his private journals, in which he referred to the Chinese during the 1997 Hong Kong handover as "appalling waxworks", he won in the High Court without having to make an appearance.

His private secretary, Sir Michael Peat, admitted at the time: "The last thing we wanted to do was to take legal proceedings against Associated Newspapers, but we have won on all the important points of principle... It was a matter of principle over a stolen document being made public."

But it was about more than that. It was about saying that if a member of the monarchy's diary isn't sacred, what is?

It seems Mr Justice Warby has similarly sympathised with the Duchess of Sussex's insistence that her letter to her father, Thomas Markle Jnr, was "personal and private".

Citing the case of Murray vs Express Newspapers, in which the author JK Rowling successfully sued over the publication of a photograph of her infant son in 2008, the judge said there was no "public interest" in the Duchess's relationship with her father.

Just as royals and celebrities aren't public property, neither are their private exchanges with their loved ones. And there is a clear difference between something being interesting to the public and in the public interest.

Hence why Charles failed in his attempt to keep his so-called "black spider memos" under wraps in 2015 following a request to publish them under Freedom of Information laws. Although the Government fought to keep the correspondence to ministers private, the Supreme Court ruled that they should be made public because they concerned affairs of state.

It didn't matter that the contents of the scrawled, hand-written notes was later described as "underwhelming" and "harmless" – unlike with his private journals, this was the heir to the throne in "advocacy" mode, and as such we had a right to know.

Quite where the line can be drawn between private matters and their public consequences remains to be seen, however.

The Royals never sued over the Camillagate or Squidgygate tapes – or the publication of love letters Diana wrote to James Hewitt. If they had, would they have won? On the face of it, Thursday's ruling would suggest that they would have had a good case for claiming misuse of private information, breach of copyright and data protection.

But unlike the Duchess's "long-form telling-off" of her estranged Dad, that intimate correspondence laid bare a deception at the heart of the Wales' "fairytale" marriage with significant constitutional consequences.

As Charles and Diana both went on to invade their own privacy by giving televised interviews about their adultery, they may not have had a legal leg to stand on. But what about Camilla, the most "private" person in the piece?

One word of caution, however. While the public will no doubt have sympathy for the Duchess in having so much Markle laundry "unlawfully" aired in public, they are unlikely to react well to her statement attacking the press for playing "games".

For as any watcher of the Royals knows, such publicity games always tend to have two players.