They added there was no fixed publication date.
A spokesperson for JK's UK publishers, Bloomsbury, added "We will announce publication of the sixth Harry Potter book as soon as we have received the final manuscript."
JK Rowling said earlier this year in a webchat for World Book Day that she was "loving writing it".
She wouldn't reveal the title. But she did say that the "wizarding world is really at war again" and that "the Muggles are noticing more and more odd happenings".
With a series as popular as Harry Potter, there are always likely to be rumours about the next book. But JK Rowling is the only person who knows the title and when it will be finished.
Although it is understood that JK is only in the early stages of writing.
So the only news that should be believed is when a release date is announced by her publishers, as they have for the first five books.
And news about the book's title should only be believed if it comes directly from her publishing company, or from an interview with JK Rowling herself. This was how she revealed that the title of Book V would be The Order of the Phoenix.