JK Rowling has been named in a lawsuit claiming she stole ideas for her wildly popular and lucrative Harry Potter books from another British author.

The estate of the late Adrian Jacobs yesterday added Rowling as a defendant in a lawsuit it filed in June against Bloomsbury Publishing for alleged copyright infringement, according to a statement by the estate's representatives, who are based in Australia.

The lawsuit, filed in a London court, claims Rowling's book Harry Potter And The Goblet of Fire copied substantial parts of Jacobs' 1987 book, The Adventures Of Willy the Wizard - No 1 Livid Land.

Jacobs' estate also claims that many other ideas from Willy The Wizard were copied into the Harry Potter books. Jacobs died in London in 1997.

Harry Potter And The Goblet of Fire is the fourth book in Rowling's series and was published in July 2000.

Sydney agent Max Markson, who is representing the trustee of Jacobs' estate, Paul Allen, said Rowling was added to the lawsuit after Mr Allen learned that the statute of limitations to sue her had not run out, as previously thought.

"I estimate it's a billion-dollar case," Mr Markson said today. "That'll be the decision of the courts, obviously."

There was no immediate comment from Rowling.

In June, Bloomsbury said the allegation that she had copied from Jacobs' work was "unfounded, unsubstantiated and untrue".

Bloomsbury said Jacobs' estate first approached the company in 2004 with its claims, but was unable to identify any text in the Harry Potter books that was copied from Willy The Wizard.

Mr Allen said the estate was also seeking legal advice on whether the Harry Potter films and soon-to-be-opened Harry Potter theme park breached copyright law.