Hackers have hijacked links on legal adult porn sites to redirect users to pictures of child abuse, said the Internet Watch Foundation (IWF).

The charity said dozens of businesses also had their servers used to host images of child sexual abuse.

The IWF said it has received 227 reports about this happening over the last six weeks, but said around 16% of men were not reporting such incidents.

IWF technical researcher Sarah Smith said: "We hadn’t seen significant numbers of hacked websites for around two years, and then suddenly in June we started seeing this happening more and more.

"It shows how someone, not looking for child sexual abuse images, can stumble across it. The original adult content the Internet user is viewing is far removed from anything related to young people or children.

"We’ve received reports from people distressed about what they’ve seen. Our reporters have been extremely diligent in explaining exactly what happened, enabling our analysts to re-trace their steps and take action against the child sexual abuse images.

"Since identifying this trend we’ve been tracking it and feeding into police forces and our sister Hotlines abroad."

The charity said the images include "some of the worst", and "were of the youngest children and the most severe levels of abuse".

Porn site visitors would click upon an image or video on the legal website, and would unwittingly be redirected to a folder containing images of child abuse, which had been placed on the hacked website without the knowledge of the website’s administrators.

In one example, a furniture business’s website was hacked and a folder containing hundreds of child sexual abuse images was uploaded.

This folder was not directly accessible from the furniture website, but from legal pornographic websites, which then link to the folder.

Representatives of Google, Microsoft, Twitter and other internet firms went to Whitehall in June to be urged by culture secretary Maria Miller to do more to clamp down on child abuse images found online.

Prime Minister David Cameron wants ISPs to block access to legal porn sites by the end of 2014 unless households request otherwise.