The Duchess of Sussex told her former aide that a handwritten letter to her estranged father was “drafted with the understanding that it could be leaked”, the Court of Appeal has heard.

Jason Knauf, who was communications secretary to Meghan and Harry until March 2019, said the duchess had indicated to him in August of the previous year that she recognised it was possible that her father, Thomas Markle, would make the letter public.

Mr Knauf’s explosive new evidence was revealed on Wednesday during the second day of an appeal by the publisher of the Mail On Sunday against a High Court judge’s decision that the publication of the letter was “unlawful”.

Lawyers for Associated Newspapers Limited (ANL) told senior judges on Tuesday that they want to rely on a recent witness statement made by Mr Knauf in their legal bid to overturn the ruling.

In his statement, Mr Knauf said the duchess had “lost confidence that the privacy of her communication with her father would be respected by him” as a result of his “increasing co-operation with reporters and photographers”.

He said that, in a series of text messages sent to him by Meghan, she had written that the “catalyst” for writing the letter to Mr Markle, 77, was “seeing how much pain this is causing H”.

Mr Knauf also said the duchess told him she was writing the letter “in part to allow the duke to demonstrate to his family that some action was being taken by the couple to stop Mr Markle from continuing to engage with the media”.

He said the texts included an early draft of the letter and that Meghan had written: “Obviously everything I have drafted is with the understanding that it could be leaked so I have been meticulous in my word choice, but please do let me know if anything stands out for you as a liability.”

He added: “On the specific issue of the letter, the duchess indicated in messages to me that she recognised that it was possible that Mr Markle would make the letter public.

“She wanted to write a letter rather than an email or text message – other options she had considered and discussed with senior Royal Household staff – as a letter could not be forwarded or cut and pasted to only share one small portion.

“As part of a series of messages on 24 August 2018, she explained that she had given careful thought to how to prevent the letter being leaked in part or in a misleading way.

“In the event that it was leaked she wanted the full narrative as set out in the letter to be understood and shared. She said she had ‘toiled over every detail which could be manipulated’.”

Andrew Caldecott QC, representing ANL, told the Court of Appeal: “The picture presented to the judge on behalf of the claimant … was that this was an entirely private letter crafted for Mr Markle’s eyes only.”

The barrister said the High Court judge had clearly taken this into account during his ruling on privacy.

He later added: “The position we now have is a different position and a more nuanced one, that the letter was written and crafted with readership in mind, and indeed she was happy for the public to read it if Mr Markle were to leak it.”

Meghan’s lawyers are opposing the appeal and ANL’s bid for the Court of Appeal to take Mr Knauf’s evidence into consideration as part of the challenge, and the duchess has provided her own statement in response.

In it, she said she had decided to write the letter after discussions with unnamed senior members of the royal family, which Mr Knauf was not privy to.

She said she did not think her father would leak the letter, as it would show him in a “bad light”.

Rebutting Mr Knauf’s evidence, she said: “The proposition that saying that I recognised that it was possible that my father would leak the letter, albeit unlikely, is the same as saying that I thought it likely that he would do so is, I would suggest, absurd.”

Meghan also said she was “eager to please” the royal family and was “especially sensitive” to their concerns about her father’s “public attacks” on them.

She added: “It is correct that, as I said in my texts to Mr Knauf, the situation was putting significant pressure on my husband, both externally and by his family, and I felt strongly that I needed to do something about it.

“I felt that, even if my attempt to stop my father talking to the media failed, at least my husband would be able to say to his family that I had done everything I could to stop it.”

The duchess also said the letter was “the only viable option” for communicating with her father, which would not have been the case before the “media intrusion” into their relationship.

The statements were disclosed to the press following representations to the court by the PA news agency.

Meghan, 40, sued ANL, which is also the publisher of MailOnline, over a series of articles which reproduced parts of a “personal and private” letter to Mr Markle in August 2018.

She claimed the five articles, published in print and online in February 2019, misused her private information, infringed her copyright and breached the Data Protection Act.

She won her case earlier this year after Lord Justice Warby ruled that ANL’s publication of her letter to her father was “manifestly excessive and hence unlawful” in a summary judgment – avoiding the need for a trial.

In March, the publisher was ordered to print a statement on the front page of the Mail On Sunday and a notice on page three of the paper stating it “infringed her copyright” by publishing parts of the letter to Mr Markle.

But the front-page statement about Meghan’s victory has not yet been published as it is on hold pending the outcome of the appeal.

The judge concluded he could make the rulings without the need for a full trial of the issues involved.

The publisher’s appeal is being heard by Sir Geoffrey Vos, Dame Victoria Sharp and Lord Justice Bean over three days, and the judges are expected to give their ruling at a later date.