"Even after a week with his dad and endlessly explaining the situation, his family seem to forget the context and revert to 'can't she just go and see him and make this stop?'" she wrote on Aug 22, 2018.
"They fundamentally don't understand, so at least by writing H [Harry] will be able to say to his family 'she wrote him a letter and he's still doing it'. By taking this form of action, I protect my husband from this constant berating, and while unlikely perhaps it will give my father a moment to pause."
The Sussexes are reported to have stayed with the Prince of Wales and Duchess of Cornwall at the Castle of Mey in Scotland in the week beginning July 28 2018. There, they are believed to have discussed what to do about Mr Markle and his public declarations.
In August 2018, the Duchess said she spoke with two senior members of the royal family – one in person and one by phone – who agreed she would write a letter.
In her witness statement to court, the Duchess explained she did not feel able to fly to see her father in person because she believed it would be impossible to reach him privately without risking bringing "yet more embarrassment" on her in-laws.
Saying "senior members of the family and their advisers expressed their concern over the public attacks" on the Royal family from Mr Markle, wanting them "stopped", the Duchess added: "I was especially sensitive to this as I had very recently married into the family and was eager to please them."
In October, at the time of the text messages, she was in the process of "meticulously" drafting a letter to Mr Markle, in which she told him of her heartbreak over his behaviour surrounding the wedding.
The letter was mentioned by one of her friends, speaking anonymously to People magazine, with extracts later published by the Mail on Sunday.
The Duchess successfully sued the newspaper's publisher for breach of privacy, copyright and data protection, but Associated Newspapers Limited is seeking to have that summary judgment overturned in favour of a trial.
Earlier this week, the court heard select details of messages between the Duchess and Mr Knauf, in which said she had "obviously" written the letter to her father "with the understanding that it could be leaked".
Saying she would address him as "Daddy" in a handwritten letter, believing it would "pull at the heartstrings" in the event that it was leaked, she added that if he shared it then "at least the world will know the truth".
At the close of the three-day hearing, the media applied for the full schedule of text messages after the Duchess herself said in her witness statement that she was "puzzled" to see them quoted in extract.
She has told the court that she had previously been unable to find the texts "because an automatic deletion system had been implemented on my phone by a cyber security specialist brought to Kensington Palace for my devices in 2016".
The texts are shown to be sent from "Tilly", a pseudonym used by Mr Knauf for the Duchess.
The messages also include emails between the Duchess, Prince Harry and Knauf in which they discuss a briefing meeting with the authors of Finding Freedom.
They show the Duchess's email signature: "Sent from my iPhone. Please excuse any technological mishaps. I'm a Luddite."
The Duchess has already won a High Court privacy battle with the Mail on Sunday after a judge found the publication of extracts of her letter to her father was unlawful.
The Mail on Sunday is seeking to overturn that summary decision, arguing that the matter should go to trial.
After a three-day hearing this week, the Court of Appeal judges will return their findings within the next few weeks.
