Be careful of the porn sites you visit. Innocent companies' websites are being hacked to serve images of child sex abuse, the Internet Watch Foundation has warned.

An innocent web user may be redirected to a site listing for furniture, but the file categories have been compromised and filled with images of terrible abuse. This puts visitors and site owners in legal danger: the defense "but I didn't know!" is not guaranteed to succeed if the unlucky netizen is subsequently charged with possession and making of indecent images.

“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," the foundation's technical researcher Sarah Smith said in a statement.

“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."

"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," Smith said.

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

The foundation said that in all cases it had been able to get the sick material removed.