The father of murdered schoolgirl Tia Sharp has said he backs a plan for websites to be told to block certain search terms and warn people when they try to view illegal content, as part of a crackdown on child abuse images.
Steven Carter told Sky News he believes Stuart Hazell was "fuelled" by images of abuse and violence he viewed before he killed the 12-year-old.
He said Tia's death may have been prevented if there was a governing body or more policing to stop the availability of indecent images online.
Mr Carter met the Prime Minister at Downing Street last week with the parents of April Jones to discuss the issue.
David Cameron is to use a major speech to tell companies they have a "moral duty" to protect youngsters.
The Prime Minister will call on websites including Google, Bing and Yahoo! to adopt a blacklist of words and phrases drawn up by the Child Exploitation and Online Protection Centre (Ceop) within months.
He will also recommend splash screens that tell people they could lose "their job, their family, even access to their children" by viewing indecent images online.
The pages could redirect people to the charity Stop It Now's website in an effort to help change people's behaviour.
"There are some searches which are so abhorrent and where there can be no doubt whatsoever about the sick and malevolent intent of the searcher that there should be no search results returned at all," Mr Cameron will say.
"I have a very clear message for Google, Bing, Yahoo! and the rest. You have a duty to act on this - and it is a moral duty.
"You are not separate from our society, you are part of our society, and you must play a responsible role in it.
"This is quite simply about obliterating this disgusting material from the net - and we will do whatever it takes."
The Prime Minister is expected to warn companies that "legislative options" could be used to force them to comply if they have not made progress on a blacklist before October.
He told the BBC's Andrew Marr show that this further step "might take a bit of an argument but it's an argument on behalf of Britain's parents and children that I am prepared to have".
He said international collaboration would be needed to make companies aware of their responsibilities.
Mr Cameron will also urge firms to hold hackathons - events which allow software experts to collaborate on projects - to produce results.
Pressure to prevent people accessing indecent images of children has increased following two high-profile murder trials.
Mark Bridger, who killed April Jones, was found to have accessed offensive images, including some of children.
Experts argued there was a clear link between their obsessions and their actions.
Labour Shadow Home Secretary Yvette Cooper said the PM isn't going far enough to tackle online child abuse.
"David Cameron said he would make sure the police had the resources. But the truth is that Theresa May has cut by 10% the resources for the Ceop - which has identified 50,000 cases of British residents accessing child abuse online but only around 2000 were pursued last year," she said.
"And the failure of the Government to develop workable proportionate communications data plans means Ceop often can't find out who is responsible for the IP addresses used for abusive child images.
"Companies do need to do much more - as Labour called for in our Parliamentary debate in June. But Ministers need to demonstrate that there will be Government action too - including quickly moving to ban depiction of rape in extreme porn as other countries have done."
Last month, the four main UK internet service providers agreed to donate an extra £1m to the Internet Watch Foundation to help its work with Ceop.
In 2012, the Government abandoned proposals for an "opt-in" system that would have forced users to contact their internet service provider in order to gain access to adult content.