Harry Potter author JK Rowling has said she will ask a court to dismiss plagiarism claims against her popular series of books, branding the allegations "unfounded" and "absurd".

Multi-millionaire Rowling has been named in a lawsuit claiming she stole ideas for her stories about a young wizard from another British author.

"We will be applying to the court immediately for a ruling that the claim is without merit and should therefore be dismissed without delay," she said Thursday.

Representatives of the estate of the late Adrian Jacobs claim Rowling's book Harry Potter And The Goblet of Fire copied parts of one of Jacobs' books about the character Willy the Wizard.

On Wednesday they added Rowling as a defendant to an ongoing legal claim against Bloomsbury, her British publisher.

Rowling hit back, saying she had never heard of Jacobs or read the book before the plagiarism claims emerged.

"I am saddened that yet another claim has been made that I have taken material from another source to write Harry," she said.

"The fact is I had never heard of the author or the book before the first accusation by those connected to the author's estate in 2004; I have certainly never read the book."

"The claims that are made are not only unfounded but absurd and I am disappointed that I, and my (British) publisher Bloomsbury, are put in a position to have to defend ourselves," she added.

The lawsuit concerns Jacobs' book The Adventures Of Willy the Wizard No 1: Livid Land.

The estate first came forward with its claim in 2004 but Bloomsbury said it had been unable to identify any text in the Potter books that had been copied.

The fortunes of Rowling, 44, from her books about the boy wizard stand at 499 milion pounds ($A853.71 million), according to last year's Sunday Times Rich List.