THE MAIL on Sunday has defended publishing a letter she sent to her estranged father arguing there is a "huge legitimate public interest" in the Royal Family and its "personal and family relationships".

The paper has published its defence after the Duchess of Sussex decided to sue the newspaper and its parent group for publishing the handwritten letter she wrote to her estranged father Thomas Markle in 2018.

Her claims include misuse of her private information, selective editing of the letter and breach of copyright.

The Mail on Sunday rejects all claims.

Her decision came in October as Prince Harry launched an extraordinary and highly personal attack on the British tabloid press and its treatment of his wife, saying he could no longer be a “silent witness to her private suffering”.

READ MORE: Queen 'supportive' of 'new life' for Harry and Meghan ...

Emphasising his respect for the importance of “objective, truthful reporting”, he accused parts of the media of “waging campaigns against individuals with no thought to the consequences” and compared the treatment of Meghan to coverage of his mother, Princess Diana.

HeraldScotland: Meghan and Harry

The Mail on Sunday has argued in a court document that members of the Royal Family, including Meghan, "rely on publicity about themselves and their lives in order to maintain the privileged positions they hold and promote themselves".

It said the duchess "did not have a reasonable expectation of privacy that the contents of the letter were private and would remain so".

In the 44-page legal filing the paper says the letter was not a protected "original literary work" but a recounting of existing facts.

While the duchess claimed it infringed her data rights, the newspaper says the data was not sensitive and concerned topics she had put in the public domain And it said extracts it released accurately conveyed the tone, content and meaning of the letter.

"There is a huge and legitimate public interest in the Royal Family and the activities, conduct and standards of behaviour of its members," the paper said.

"This extends not merely to their public conduct, but to their personal and family relationships because those are integral to the proper functioning of the monarchy."

As evidence that it did not infringe her privacy, the paper says the letter was "immaculately copied" in the duchess's "elaborate handwriting", arguing that this care in its presentation meant she anticipated it would be seen and read by a wider audience.

The move came after Harry and Meghan announced that they want to "step back" from being full-time working royals. The Queen has agreed a "period of transition" in which Prince Harry and Meghan will be in Canada and the UK.

READ MORE: Harry and Meghan to 'step back' as senior royals 

Libel lawyers Schillings were brought in in October, to file a High Court claim against the Mail on Sunday and its parent company Associated Newspapers over the alleged misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.

HeraldScotland:

The firm in a statement talked of the "intrusive and unlawful publication of a private letter written by the Duchess of Sussex, which is part of a campaign by this media group to publish false and deliberately derogatory stories about her, as well as her husband".

It added: “Given the refusal of Associated Newspapers to resolve this issue satisfactorily, we have issued proceedings to redress this breach of privacy, infringement of copyright and the aforementioned media agenda.”

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

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.”

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.