Last October, Meghan began legal action against the Mail on Sunday tabloid with her lawyers describing the publication of her letter as part of a "campaign by this media group to publish false and deliberately derogatory stories about her, as well as her husband".

In a statement at the time, husband Prince Harry called the coverage of his wife by the British press "bullying", likening it to the hounding his mother Princess Diana had suffered before her death in a Paris car crash in 1997.

The couple said the Mail on Sunday had printed articles based on a private and confidential letter to her father, in breach of her human rights and copyright, accusations the paper rejected saying her royal position meant the stories were valid.

"There is a huge and legitimate public interest in the royal family and the activities, conduct and standards of behaviour of its members," lawyers for the paper said in documents submitted to London's High Court this week.

"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."

The Daily Mail reported on Wednesday that Markle was prepared to testify against her. A spokeswoman for Harry and Meghan was not immediately available for comment.

The disclosure comes after 93-year-old Queen Elizabeth reluctantly agreed on Monday to grandson Harry and Meghan's request to step back from their senior royal roles, and allow them to split their time between Canada and Britain and become financially independent.

Meghan, who is currently in Canada with the couple's baby son Archie, made her first public appearance since that decision at a woman's centre in Vancouver on Tuesday.

Her father became the focus of huge media attention in the run-up to Meghan and Harry's star-studded 2018 wedding at the queen's home Windsor Castle.

The former lighting director for US TV soaps and sitcoms pulled out days beforehand after undergoing heart surgery and it also emerged that he had staged photos with a paparazzi photographer. He has since given a number of interviews to British tabloids, saying he had been ignored by his daughter.

In its 44-page defence, the newspaper said that unnamed friends of Meghan had put her version of events in interviews with the US magazine People and Markle had the right to put his side.

"Thomas Markle had a weighty right to tell his version of what had happened between himself and his daughter including the contents of the Letter," the Mail's defence said.

Australian Associated Press