David Cameron will tell internet giants including Google they have a "moral duty" to do more to tackle child abuse images found by using their websites.
In a major speech he will call for search engines to block any results being displayed for a "blacklist" of terms compiled by the Child Exploitation and Online Protection Centre (Ceop).
The Government has been involved in negotiations with technology firms over the best way to crack down on child abuse, and the main service providers have agreed to introduce ''splash pages'' which tell people if they are attempting to view illegal images.
But the Prime Minister will call on firms to go further, with splash screens warning of consequences "such as losing their job, their family, even access to their children" as a result of viewing the content.
Calling for internet providers to "step up to the plate" he will say the warning pages should also direct people to the charity campaign Stop It Now, in an effort to help change people's behaviour. The Prime Minister will also warn firms that "legislative options" could be used to force them to comply if they have not made progress on a black list by October. He will say: "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 reject attempts by the internet giants to claim that the task is beyond their capability as he sets the October deadline. Saying the Government will do "whatever it takes" to tackle the problem, Mr Cameron will urge the firms to hold "hackathons" - events where software experts collaborate - to produce results.
"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," he will say. "I have called for a progress report in Downing Street in October, with the search engines coming in to update me. The question we have asked is clear: If Ceop give you a blacklist of internet search terms, will you commit to stop offering up any returns to these searches?
"If in October we don't like the answer we're given to this question, if the progress is slow or non-existent then I can tell you we are already looking at the legislative options we have to force action. And there's a further message I have for the search engines. If there are technical obstacles to acting on this, don't just stand by and say nothing can be done; use your great brains to help overcome them.
"You're the people who have worked out how to map almost every inch of the earth from space, who have developed algorithms that make sense of vast quantities of information. You're the people who take pride in doing what they say can't be done. You hold hackathons for people to solve impossible internet conundrums. Well - hold a hackathon for child safety. Set your greatest brains to work on this. 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."
Pressure to act against online child porn 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.