Rowling will try to block the publication of The Harry Potter Lexicon by RDR Books.
The US publisher has suggested that Potter fans could buy both the lexicon and the encyclopaedia. But Neil Blair, copyright lawyer for Rowling's literary agency Christopher Little, told Third Sector that RDR's argument was "naive and wrong" because not everyone would be able to afford both.
"For every copy of the American book bought, that could be £20 or £30 that's not going to the good causes that Rowling is supporting," he said. "Over and above trying to protect JKR's copyright, the rationale behind taking a stand here is that the money generated from the sale of this book would go to charities, and anything that might limit that revenue is regrettable."
So far, Rowling's other companion books, Quidditch Through The Ages and Fantastic Beasts, have generated more than £16m between them for Comic Relief. "It is clear that the definitive Harry Potter encyclopaedia could well raise significantly more than that," Blair said.
For Comic Relief, Rowling donated her royalties and asked publishers, printers, paper suppliers and retailers to contribute.
In December, one of seven handwritten copies of Rowling's book The Tales of Beadle and the Bard was sold at Sotheby's, raising £1.95m for the child abuse charity Children's Voice Campaign.