Girl Online, Zoella's debut novel, was written with "help" from her publisher, the YouTuber has admitted.
In a post shared on Twitter, she said: "Of course I was going to have help from Penguin's editorial team in telling my story.
"The story and characters of Girl Online are all mine."
Another author, Siobhan Curham, who is thanked in the book's acknowledgements, has been responding to Twitter comments about the Girl Online revelations.
It has not been suggested by either the publisher, or Ms Curham herself, that she wrote the book - but Ms Curham has thanked followers for their concern.