A mother has demanded Amazon take action after her 12-year-old daughter accidently found sexually explicit material on its website.
She said her daughter had typed "free Kindle books for teenagers" into a search and results included titles like Being Bad and Bad Babysitter.
The woman, who has asked the BBC to use only her forename, Nicola, said she was "gobsmacked" at the X-rated material.
Amazon said the titles were "temporarily miscategorised".
Nicola, who lives in the south of England, said her daughter was looking for free books to download for a new Kindle which she was to be given as a present.
"I'm trying to protect my teenager in every way possible," she said.
"I'm doing all the things that I ought to do and a company like Amazon is not only allowing her to access it but is actually offering it to her when she's not even looking for it.
"You don't even need the word 'free', you can just type 'kindle books for teenagers'.
"I tried Trading Standards who are going to look at it and they said if you're concerned then ring the police, who said, 'That's awful but there's not a lot we can do about it.'"
Nicola said she had attempted to contact Amazon "on numerous occasions today and yesterday".
She added: "The only people I could get through to were at their customer services in the Philippines who were at a loss to know what to do about it."
In statement, Amazon said: "These titles were temporarily miscategorised by our systems and should not have appeared under these search terms. We're fixing the error and have contacted the customer to apologise."
But John Carr, a child safety campaigner, who first blogged about this topic two years ago, told the BBC: "You can stop this but it effectively renders your Kindle unusable as you have to stop your children accessing the bookshop.
"Lots of parents buy their children a Kindle hoping up to open a world of reading. No-one suspects by giving their children a Kindle they're opening up a world of porn."
The NSPCC, which has guidelines on how to protect children online, said: "Amazon state that under 18s should not use the site without parental supervision.
"However, this should be made much clearer on sign-up so that parents can take steps to prevent children viewing or purchasing age-inappropriate content."
Amazon ran into similar problems in 2013, when a grandmother in Chicago, Illinois, discovered sexually explicit material after searching 'teen books for girls' while browsing for a book to give to her 12-year-old granddaughter.