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 argued Meghan gave the authors of Finding Freedom information 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 her ruling, but her lawyers could still take the case to the Court of Appeal.
The duchess is suing for breach of privacy and copyright infringement after articles reproduced parts of a letter she sent to Thomas Markle.
The publisher denies the allegations.
The High Court case is scheduled to begin in January next year.