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A Captain Underpants spin-off book has been taken off sale and children's author Dav Pilkey has apologised for its "harmful racial stereotypes and passively racist imagery".

The book, titled The Adventures of Ook and Gluk: Kung-Fu Cavemen from the Future, was first published in 2010.

In a statement, Pilkey said it was meant to "showcase diversity, equality, and non-violent conflict-resolution".

But he said he now realised it was "wrong and harmful" to Asian people.

"I hope that you, my readers, will forgive me, and learn from my mistake that even unintentional and passive stereotypes and racism is harmful to everyone," the US author wrote in .

"I apologise, and I pledge to do better."

Publisher Scholastic said it would attempt to remove the book from library and school shelves as well as shops.

The book is a graphic novel supposedly written by the two fictional friends who feature in the Captain Underpants books, and follows "cave boys" called Ook and Gluk.

Pilkey said they "save the world using kung fu and the principles found in Chinese philosophy". It had an initial worldwide print run of a million copies.

The Captain Underpants series includes 12 novels and numerous spin-offs. They are hugely popular with young readers, with more than 80 million copies sold around the world, but are often among the American Library Association's most complained-about books because of their violent imagery.

Scholastic said it had halted publication of the book in question on 22 March with Pilkey's "full support".

"Together, we recognise that this book perpetuates passive racism," the company said in a statement. "We are deeply sorry for this serious mistake."

Pilkey said he would donate the advance and royalties he received for the book to charities that provide books to children in underserved communities, promote diversity in children's book, and fight anti-Asian prejudice.

The move comes amid an examination of anti-Asian sentiment in the US following a rise in harassment and hate crimes, and in light of the murders of six Asian women in a shooting in Atlanta earlier this month.

It also comes as some children's books and characters are being reassessed. In early March, the company that preserves the legacy of author Dr Seuss said six of his books would no longer be published because of racially insensitive imagery.