Meghan Markle wins privacy claim against UK news group
The Duchess of Sussex, who is married to Queen Elizabeth II's grandson Prince Harry, brought the case against Associated Newspapers after extracts of the 2018 letter appeared in 2019.
Judge Mark Warby ruled that Meghan had a "reasonable expectation that the contents of the letter would remain private".
The letter to Thomas Markle was written a few months after Meghan married Harry, and asked her father to stop talking to tabloids and making false claims about her in interviews.
The newspaper group's defence team argued the publication was to correct inaccuracies in a previous article in the US magazine People.
But the judge said the extracts published were "manifestly excessive and hence unlawful".
Meghan, 39, welcomed the ruling, and thanked the court for holding Associated to account for what she said were "illegal and dehumanising practices".
She accused the Mail on Sunday weekly, its daily sister title the Daily Mail and the MailOnline website of exploiting individuals like "a game".
"For me and so many others, it's real life, real relationships and very real sadness. The damage they have done and continue to do runs deep," she added.
"The world needs reliable, fact-checked, high-quality news. What the Mail On Sunday and its partner publications do is the opposite."
The judge said a separate claim for breach of copyright should go to a partial trial to determine authorship of the letter, after claims that royal officials helped Meghan to draft it.
Warby said suggestions that she was not the sole author of the letter, and that the Crown could share the copyright, "cannot be described as fanciful". They warranted further investigation, he said.
It was not immediately clear if that meant Meghan would have to give evidence.
The news group said it was "very surprised" by the judgment and "disappointed at being denied the chance to have all the evidence heard and tested in open court at a full trial".
A decision on whether to appeal was being considered, it added.
The publishers had argued that witnesses were needed to "shed light" on whether Meghan was planning for the letter to be made public a part of a "media strategy".
But Meghan's lawyers described the letter as "self-evidently private" and denied she intended the letter to be made public at any point.
They also denied she collaborated with the authors of a recent biography on her life with Harry, which also contained partial extracts of the letter.
Meghan and Harry quit frontline royal duties in March last year and now live in California.
They have launched a number of legal cases against news outlets alleging invasion of privacy, including over paparazzi shots of their son Archie.
This has attracted criticism from some, as the couple are also launching themselves into the public eye with high-profile projects such as a Spotify podcast where Archie made a brief appearance.