The children’s publisher Puffin is hurriedly contacting UK schools and local authorities after it discovered that a website address included in a popular children’s book series had been hijacked by a pornography site.

The publisher has urged school libraries to remove the books after a website address printed in the Spy Dog, Spy Pups and Spy Cat series by Andrew Cope was redirected to unrestricted adult content.

Puffin, an imprint of Penguin Random House, said it had “immediately paused” sales of the affected books, aimed at seven- to 12-year-olds, and was working to have libraries remove the books from their shelves.

A number of schools in England reported that they had received emails alerting them to the danger, including a warning that the new content included “inappropriate material featuring children’s characters”.

One school in West Sussex wrote to parents: “We have been made aware of a safeguarding alert linked to the children’s book series Spy Dog/Pups and Spy Cat written by author Andrew Cope.

“These books contain a link to a website to find out more about the character (identified on the back or inside page). This link has now been compromised and takes users to a pornographic website with no age verification.”

A statement issued on behalf of Cope and Puffin said: “In some editions of the Spy Dog, Spy Cat and Spy Pups series there is a reference to the author Andrew Cope’s former website for the series which was owned and managed by him.

“We understand an unconnected third party has very recently taken control of the domain name and is using it to display a different website with inappropriate adult content. This website is not associated with Puffin or Andrew Cope. We are asking people not to visit the website and ensure that children do not visit it either.

“We are taking this extremely seriously and are acting on this issue as a matter of urgency through the appropriate channels to get this website removed. This is an in-depth and complex legal process and will take time. We have immediately paused the sale and distribution of the books whilst we deal with this matter.”

skip past newsletter promotion

Discover new books and learn more about your favourite authors with our expert reviews, interviews and news stories. Literary delights delivered direct to you

Privacy Notice: Newsletters may contain information about charities, online ads, and content funded by outside parties. If you do not have an account, we will create a guest account for you on theguardian.com to send you this newsletter. You can complete full registration at any time. For more information about how we use your data see our Privacy Policy. We use Google reCaptcha to protect our website and the Google Privacy Policy and Terms of Service apply.

Spy Dog, the first in the series, was published in 2005, introducing Lara, a dog trained by the secret service and working undercover with an unwitting family. Another 11 stories followed, and from 2009 until 2015 further series of Spy Pups and Spy Cat spin-offs followed.

Hampshire’s school library service sent out a warning to families about the “unsafe material” on the website. “If you have any of these books in your house, you might wish to take a look and take the appropriate action,” it said.