Meghan Markle's dad set to testify in court against his own daughter in 'trial of the century

Thomas Markle and his daughter Meghan Markle.

Meghan Markle could be pitted against her father in the witness box if he testifies against her in what could be the 'trial of the century'.

In October, The Duchess of Sussex took legal action against the Mail on Sunday over its decision to publish a highly personal letter which she penned for her dad.

But the estranged father and daughter appear to be on a collision course after reports that the newspaper signed up Meghan’s father, Thomas Markle, as its star witness.

The Mail on Sunday will argue there is a huge public interest in the personal relationships of members of the royal family as part of its defence, court documents show.

The newspaper also says royals "rely on publicity about themselves and their lives to maintain the privileged positions they hold".

In February, the Mail on Sunday published extracts of Meghan's handwritten letter to her estranged father Thomas Markle.

In one extract, published by the newspaper, the duchess 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."

Meghan's lawyers filed a High Court claim against the paper and its parent company Associated Newspapers over the alleged misuse of private information.

But in its legal defence 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", according to the BBC.

It comes just 24 hours after the Queen agreed Harry and Meghan could step back as senior royals and begin a "new life" as an "independent" family.

Buckingham Palace also confirmed the Sussexes would begin a "transition period" in which they would split their time between the UK and Canada, where the Duchess is currently with their son Archie.

Mr Markle was caught up in controversy in the build-up to the 2018 wedding after he allegedly staged paparazzi photographs of himself and then began commenting regularly to entertainment website TMZ about his contact with his daughter.

When the legal action was announced in October, Harry claimed the alleged unlawful publication of the private letter was done in "an intentionally destructive manner" to "manipulate" readers.

In a statement released at the time, a spokeswoman for law firm Schillings claimed the "intrusive" publication of the letter was part of Associated Newspaper's campaign to write "false and deliberately derogatory stories about" Meghan, "as well as her husband".

But the Mail on Sunday said it would stand by the story and denied editing the letter to change its meaning.