"Put simply - there needs to be a list of terms -- a black list -- which offer up no direct search returns," Cameron said, according to a draft version of the speech text published on his office's website. "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."

Arguments that a ban on searches that clearly reflect the "sick and malevolent intent of the searcher" would violate freedom of speech will not be accepted, he said.

Cameron said that more general search queries like "child sex" should return options like: "Do you mean child sex education?" or "Do you mean child gender?" In addition, "abhorrent" combinations of search keywords that are clearly related to child abuse should return no result at all, Cameron said.

Internet search providers have until October to answer whether they are willing to commit to censor results for search terms on a blacklist compiled by the Child Exploitation and Online Protection Centre (CEOP Center), a law enforcement organization affiliated with the U.K.'s Serious Organised Crime Agency (SOCA) that is dedicated to fighting the sexual abuse of children.