Thomas Markle could testify against daughter Meghan in her legal battle with the Mail on Sunday as High Court documents revealed he forms part of the newspaper's defence.
The publication and its parent company Associated Newspapers are accused of unlawfully publishing a handwritten note from the Duchess of Sussex to her father from August 2018.
However the newspaper will argue it was justified because there is a "huge and legitimate" public interest in the "personal relationships" of members of the Royal Family.
Legal documents revealed that the Mail on Sunday will rely on evidence from Mr Markle, including that he "had a weighty right to tell his version of what had happened between himself and his daughter including the contents of the letter".
So if the case goes to trial, both Meghan and her father could be called to testify against one another, with the Daily Mail reporting Mr Markle would be prepared to face her in court.
Schillings, the law firm representing Meghan, filed the High Court claim against the paper in October, alleging misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.
But the paper claims the duchess "did not have a reasonable expectation of privacy that the contents of the letter were private and would remain so".
More from UK
The Mail on Sunday also argues the publication of the letter was in response to a "one-sided" interview with five of Meghan's friends in People Magazine in February 2019.
The People article said the duchess had written to Mr Markle: "Dad, I'm so heartbroken, I love you, I have one father. Please stop victimising me through the media so we can repair our relationship".
However the paper's defence said Meghan's note was far from being "a loving letter aimed at repairing their relationship" and "was an attack on Mr Markle".
"Amongst other things, she accused him of breaking her heart, manufacturing pain, being paranoid, being ridiculed, fabricating stories, of attacking Prince Harry and continually lying".
Extracts of the letter were published in the Mail on Sunday in February 2019.
In one extract published by the newspaper, Meghan wrote: "Your actions have broken my heart into a million pieces – not simply because you have manufactured such unnecessary and unwarranted pain, but by making the choice to not tell the truth as you are puppeteered in this. Something I will never understand."
When the legal action was announced in October, Prince Harry claimed the alleged unlawful publication of the letter was done to "manipulate" readers in an "intentionally destructive manner".
A Schillings spokeswoman claimed at the time that the "intrusive" publication of the letter was part of Associated Newspapers' campaign to write "false and deliberately derogatory stories about" Meghan, "as well as her husband".
The Mail on Sunday said it would stand by the story and has denied editing the letter to change its meaning.