Video report by ITV News Royal Editor Chris Ship
But the 44 pages of defence that the newspaper's owners have now lodged show just how personal this matter is for the Duchess of Sussex.
There are some suggestions that both will have to appear in court to testify against each other – although their evidence could also be submitted in written form.
Associated Newspapers, which owns the Mail on Sunday, says that Meghan did not have “a reasonable expectation of privacy” when she sent the personal letter to Thomas Markle in August 2018.
The newspaper says Mr Markle gave them the letter in February 2019.
Its defence case rests on the fact that Meghan is a high profile member of the Royal Family, and previously an actor, and there is a “huge and legitimate public interest” in her activities.
It claims she “enjoys immense privilege and wealth” and leads a lifestyle of private jets and historic houses that is only available to “those of extreme wealth or elite connections".
And because of the position she holds in the Royal Family, Associated Newspapers says the public interest "extends not merely to their pubic conduct, but to their personal and family relationships".
In fact, the defence papers go as far as to claim that we care more about them than our own families: “The public are encouraged to and do take an interest in the royal family in a way which may exceed the interest that they would take in members of their own extended family.”
It was at the end of the Sussexes’ tour of Africa last October, when the couple announced that Meghan was suing the Mail on Sunday for damages over the publication of the letter to her father.
The relationship between father and daughter had deteriorated in the weeks before the 2018 wedding, after Mr Markle had cooperated with paparazzi photographers and staged pictures of himself preparing for Meghan and Harry’s big day.
Harry has still never met his father-in-law and Mr Markle has never seen his grandson, Archie.
The newspaper’s lawyers argue that Meghan did not have a “reasonable expectation of privacy that the contents of the letter were private and would remain so” because Meghan “knew that her father had spoken to the media previously about their relationship”.
Thomas Markle had been speaking to newspapers and television shows in the UK and the US in the months after the wedding.
They argue this on the grounds that friends of Meghan had spoken anonymously to People magazine in February 2019 and they had revealed the existence of the letter.
It is not known whether or not Meghan had authorised her friends to speak to People magazine.
A judge will now have to determine how robust the Mail on Sunday’s defence is.
Whatever the final ruling, this case has already exposed the depth of the rift that has opened up between Meghan and her father since she married into the Royal Family.