Rowling has maintained that "Harry Potter and the Deathly Hallows" which will be published July 21, is the last in a seven-book series, but she is keeping the door open just a narrow gap.
She issued a statement through her publisher Bllomsbury, saying "never say never" in response to the "Save Harry" campaign after having made similar remarks during a BBC television interview on Friday.
"I think that Harry's story comes to quite a clear end, sadly," Rowling said.
"But I've always said that I wouldn't say never. I can't say I'll never write another book about that world just because I think, what do I know in 10 years' time...? But I think it's unlikely."
Rowling had previously disclosed that two characters will die, fuelling speculation that one of them will be Harry himself.
But fans are not deterred.
In a world of magic and wizardry, Harry could be brought back from the dead in an eighth book, they suggest.
Bookseller Waterstone's has launched the Save Harry campaign.
The petition at www.waterstones.com/saveharry launched Monday reads: "We, the undersigned, petition J.K. Rowling to write more new adventures for Harry Potter and his friends no matter what happens at the end of 'Harry Potter and the Deathly Hallows.'"
Waterstone's argued that there is a precedent for resurrecting a literary hero, pointing to the case of Sir Arthur Conan Doyle and Sherlock Holmes.
In 1893, weary of his work, Conan Doyle wrote "The Adventure of the Final Problem," in which Holmes apparently falls to his death during a violent struggle with his nemesis, Professor Moriarty.
The fictional detective turned out to have survived the fall after a public clamour persuaded Conan Doyle to resurrect him.
In presenting its petition, the Waterstone's website says: "Will July 21 mark the end of Harry Potter?
"J.K. Rowling says so, but history is full of magnificent returns from the dead, so there is no reason why, with your support, we can't persuade her to write more adventures for Harry and the Hogwarts gang.
"If Sherlock Holmes, Dennis Watts, Superman, and Bobby Ewing can all come back from the dead then surely reincarnation shouldn't be a problem for the world's most powerful boy wizard -- and that's presuming that he dies at the end of 'Harry Potter and the Deathly Hallows.'
"If he survives, then bringing him back should be even easier."
Dennis Watts and Bobby Ewing were soap opera characters in, respectively, Britain's "Eastenders" and the US's "Dallas", who were brought back to life after lengthy periods in which viewers had believed they were dead.
The forthcoming "Harry Potter and the Deathly Hallows" will cap a series that has sold 325 million copies worldwide and has been translated into 64 languages, as well as being converted into a successful film franchise.
The books have made Rowling an estimated one billion dollars (733 milion euros). The first Potter volume appeared in 1997.
Those involved in the printing of the new book have been asked to keep its plot a watertight secret until July 21, when the English version makes its debut.