Meghan, the Duchess of Sussex, kept a copy of a highly personal handwritten letter she sent to her father Thomas Markle, suggesting she may have correctly feared it would later be leaked to the media.

Court filings seen by the Guardian show that the duchess has a full record of the correspondence, including unpublished sections, which is now being used to assist her legal case against the Mail on Sunday for copyright infringement and invasion of privacy.

The duchess’s lawyers also allege that the letter was selectively edited by the Mail on Sunday to remove paragraphs which were highly critical of the British tabloid media, as well as comments which showed that Meghan retained genuine concern for her father’s wellbeing.

The documents also state that the Mail on Sunday did not request a comment from the royal family or inform Meghan that they were preparing to print the letter, in an apparent attempt to avoid an application for an injunction.

Meghan, who is described by her solicitors as a “well-known American actor, business entrepreneur, and women’s rights activist” is claiming the Mail on Sunday invaded her right to a private life under the Human Rights Act. She is also claiming damages over allegations that the newspaper did not handle her personal information correctly under data protection legislation and breached copyright laws by publishing large chunks of the letter without her permission.

“Although the claimant is well known to the public, the details of her feelings about her relationship with her father are not a matter of public interest, nor do they relate to her public profile or work,” her lawyers state.

Her husband, Prince Harry, last month warned of newspapers “waging campaigns against individuals with no thought to the consequences” and compared the treatment of Meghan to coverage of his mother, Princess Diana. He also launched legal proceedings against the publishers of the Sun and the Mirror for alleged phone hacking, as part of the couple’s all-out legal war on the UK’s tabloid media.

Meghan wrote to her father in August 2018 begging him to stop talking to reporters and saying he had broken her heart “into a million pieces” with his constant media interventions during her relationship with Prince Harry. The letter was posted via FedEx by Markle’s Los Angeles-based representative.

Earlier this year Thomas Markle handed the letter to the Mail on Sunday, which ran excerpts of the correspondence as a “world exclusive”, alongside his comments about being cut out of the relationship with his daughter.

The Mail on Sunday has stood its ground since the first legal letters were sent in February, with the case currently heading to court. After the existence of the legal action was revealed, the Mail on Sunday ran a staunch defence of its reporting, saying Thomas Markle had originally intended to keep the existence of the letter private.

According to Markle, he decided to go public after reading a lengthy piece in the February edition of the US magazine People, with quotes attributed to “close friends of Meghan”, which the royal household has never denied was authorised by the duchess.

He said it misrepresented the reality: “I decided to release parts of the letter because of the article from Meghan’s friends in People magazine. I have to defend myself. I only released parts of the letter because other parts were so painful. The letter didn’t seem loving to me. I found it hurtful.”

The Mail on Sunday has always maintained that it did not edit the letter in any way that changed its meaning. The newspaper has also made clear that it did not pay Thomas Markle when he provided the letter to the newspaper.

Meghan also decided to abandon the law firms traditionally used by the royal family and employ the services of reputation management specialists, Schillings, who pride themselves on fiercely representing their clients and often keep stories out of the newspapers.

The Mail on Sunday declined to comment.