The Mail on Sunday can use a recent biography of the Duke and Duchess of Sussex in its defence, in a High Court privacy claim over the publication of a letter to Meghan's estranged father.
The paper said Meghan gave information to the Finding Freedom authors in order to set out her own version of events.
But Meghan's lawyers said accusations they "collaborated" with the authors were a "conspiracy theory".
The judge said the publisher can amend its defence to rely on the biography.
Judge Francesca Kaye refused the duchess permission to appeal against the ruling, but her lawyers can still take the case to the Court of Appeal.
The duchess is suing for breach of privacy and copyright infringement after articles in February 2019 reproduced parts of a handwritten letter she had sent to Thomas Markle.
Meghan claims the letter was "private and confidential" and "detailed her intimate thoughts and feelings about her father's health and her relationship with him at that time".
The Mail's publisher denies the allegations and argues the duchess had no reasonable expectation of privacy and anticipated publication of the letter.
In the recent biography Finding Freedom, Omid Scobie and Carolyn Durand describe a culture of increasing tension between the Sussexes and other members of the Royal Family.
The Mail claimed that Meghan gave the authors information about the letter to Mr Markle "in order to set out her own version of events in a way that is favourable to her".
But Meghan's lawyers argued that references to the letter in the book were simply "extracts from the letter lifted from the defendant's own articles".
The Duke and Duchess of Sussex issued a statement at the time of the book's publication in July to say they had not been interviewed for the book "and did not contribute" to it.
The couple are now based in California, having stepped back as senior royals at the end of March.
The High Court case is scheduled to begin in January next year.