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Meghan Markle has waged a legal battle against a British newspaper for publishing a confidential letter she wrote to her father, Thomas Markle, at the height of their fractured relationship.

The United Kingdom's High Court held its first hearing in the privacy lawsuit last month and a legal expert told Fox News the courtroom drama will become heightened if and when the Duchess and her estranged father testify against each other.

"The big battle is going to be between Meghan and her dad. That will be horrible for both of them, I suspect," Amber Melville-Brown, head of media and reputation at legal firm Withers, told Fox News in an interview.

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Meghan Markle may be forced to testify against her father, Thomas Markle, in her privacy lawsuit in the U.K.'s High Court. (Getty/The Mega Agency)

"This case is all about if [Meghan] had a reasonable expectation of privacy. This means she'll have to give evidence as to what she did with the letter and the impact [the publishing of it] had on her. The court won't know how deeply distressing this is to her unless she gives evidence," explained Melville-Brown, who is dually-certified in both the U.K. and New York.

It's a bold move for the Duchess of Sussex, who, alongside husband Harry, has voiced disdain for the couple's personal matters and recent departure from the royal family becoming tabloid fodder. Last month, the couple announced they will no longer cooperate with certain British tabloids.

She is suing the Mail on Sunday's publisher, Associated Newspapers, for copyright infringement and misuse of private information for releasing contents of the letter. She also argues the publisher violated the U.K.'s data protection act.

The veteran privacy lawyer stressed the importance of protecting the privacy of clients but worries that Meghan Markle has already "overbaked the litigation cake" by taking news coverage too personally. Melville-Brown says that the lawsuit may lead to a win when it comes to copyright infringement, but it won't put an end to the public's interest in her private life.

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Meghan, Duchess of Sussex and Prince Harry, Duke of Sussex attend The Endeavour Fund Awards at Mansion House on March 5 in London, England.  (Chris Jackson/Getty Images)

"Meghan and Harry have made it personal for them. Harry already had a completely understandable hatred of the press because of the role he believes they played in his mother's death. It's clear the couple has gotten to the end of their tether, but making the case personal doesn't mean they can win the war," the lawyer said.

Melville-Brown warned that in order for Markle to succeed in her infringement and privacy case, she will have to be prepared "for all the publicity that comes with it."

"When you bring an action for privacy invasion it's sometimes a dangerous thing to do. Meghan and Harry have painted themselves into a corner because, as they're attempting to gain control of their image and protect their privacy, she will also have to speak to it," the lawyer furthered. "She is opening up the possibility of being cross-examined in open court."

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Markle had already suffered a setback in the case at the beginning of this month. The judge presiding over the case ruled in favor of Associated Newspapers by striking the Duchess' arguments that the publication acted dishonestly and attempted to further rile up conflict between Markle and her dad.

Meghan Markle was a Hollywood actress before she became a member of the British royal family. (Lars Niki/Corbis via Getty Images)

Markle's attorneys told Fox News in a statement that they were "surprised" by the judge's May 1 verdict but vowed the case will continue in court with the arguments of copyright infringement and misuse of privacy.

Melville-Brown told us that Markle's first loss in court was "more than a slight setback."

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"The first win in the war is an important one," she said. "Psychologically, it's very important as a litigator because it sets the tone for the case."

If Markle wants to succeed in court, the expert said she will have to keep her arguments "very narrow" and leave her personal strife with the media out of it.