Dozens of websites are being hacked to host images of child sexual abuse which are being inadvertently viewed by internet users, a charity has warned.
The Internet Watch Foundation (IWF) said it had received 227 reports of hijacked websites in the last six weeks.
The charity said legal pornographic sites had also been attacked to redirect visitors to the illegal material.
Susie Hargreaves, chief executive of the IWF, told BBC Radio Five Live: "It means for the person whose accidentally stumbled across it, they're seeing the worst of the worst without any idea of how it actually happened."
Pressure to act against online images of child abuse has increased in recent months following high-profile murder trials.
Mark Bridger, who killed April Jones, and Stuart Hazell, murderer of Tia Sharp, were both found to have accessed child and violent pornography and some experts argue there is a clear link between their obsessions and their actions.
David Cameron has threatened to impose tough new laws on internet giants if they fail to blacklist key search terms for horrific images by October as part a crackdown on online porn.
Possessing violent pornography containing simulated rape scenes will be made a crime in England and Wales and videos streamed online in the UK will be subject to the same restrictions as those sold in shops.
But Jim Gamble, the former head of CEOP, the Child Exploitation and Online Protection agency, said more investment was needed in law enforcement as child abuse images were often hidden by extended security measures.
"These people will go to great lengths to hide who they are and to distribute to their network," he told BBC Radio Five Live.
"That's why I was deeply concerned when people were presenting the search initiative from the Prime Minister's office as a solution to child protection.
"These people aren't going online and hitting child abuse images in a one click search. They're going through these very extended security measures, they're hiding and sharing within specific groups.
"We have to get to the back end of this, the root cause, which is people. That's got to be an investment in law enforcement."