Harry - who shares son Archie with Meghan - noted that the positive publicity they received on their royal tour of South Africa indicated the Press's "double standards."
The Sun reported that the royal couple were criticised over the timing of the statement, which was released at the end of the couple's royal tour.
Meghan and Harry were said to have followed "specific legal advice" and a spokesman for Prince Harry said the case had been in the works for "many months."
However, according to the publication, the Duke had another six years to make the claim, and experts believe the law firm Schillings filed it at the end of the royal tour to "cause maximum impact."
"This is not the done thing on a royal tour, when the principals are representing the Queen and country," a royal source told The Sun.
“I can’t imagine the Queen will be very impressed.”
However, the Queen reportedly banned chat about Prince Harry and Meghan Markle altogether.
The statement on the Duke and Duchess of Sussex's website reads: "As a couple, we believe in media freedom and objective, truthful reporting. We regard it as a cornerstone of democracy and in the current state of the world – on every level – we have never needed responsible media more.
"Unfortunately, my wife has become one of the latest victims of a British tabloid press that wages campaigns against individuals with no thought to the consequences – a ruthless campaign that has escalated over the past year, throughout her pregnancy and while raising our newborn son."
Prince Harry then wrote about his fears for the Duchess of Sussex, and that the "bullying" mirrors that which happened to his mother, the late Princess Diana - who died in a car crash while being chased by the press.
He wrote: "Though this action may not be the safe one, it is the right one. Because my deepest fear is history repeating itself. I’ve seen what happens when someone I love is commoditised to the point that they are no longer treated or seen as a real person. I lost my mother and now I watch my wife falling victim to the same powerful forces."
Meghan Markle was once overheard saying that the British media 'don't make it easy' for her.
The claim is being filed against the Mail on Sunday over the alleged misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.
The Mail on Sunday publisher Associated Newspapers said it “stands by the story it published and will be defending this case vigorously”.
It added: “Specifically, we categorically deny that the Duchess’s letter was edited in any way that changed its meaning.”