U.K. Internet suppliers will have to block access to pornography unless customers opt to receive it, an effort to help parents stop children viewing unsuitable material, Prime Minister David Cameron declared Monday.

The premier said all new customers will automatically have family-friendly filters installed, and existing customers will be contacted and told they must say whether they want to turn those filters on or off. Those who don’t reply will have the filters activated.

Cameron urged search-engine providers such as Google Inc., Microsoft Corp. and Yahoo! Inc. to “step up to the plate” and stop providing results when people search for images of child abuse.

“There needs to be a list of terms — a blacklist — which offer up no direct search returns,” Cameron said in a speech in London today. “There are some searches which are so abhorrent and where they can be no doubt whatsoever about the sick and malevolent intent of the searcher. 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.”

Cameron also announced a proposal to make it a crime to possess violent pornography containing simulated rape scenes, and said Google and other search engines would be asked to block searches based on certain phrases.

Anti-pornography activists welcomed the announcement, but Padraig Reidy of free-speech group Index on Censorship said it amounted to “a kind of default censorship.”

Cameron  said he wants search engines to report to him by October on their progress.

“There’s lots of complicated and difficult questions that have to be answered and that’s why we have to work with the companies,” Cameron said on BBC Radio 2’s “Jeremy Vine Show” today. “I know it’s hard but that’s not a reason for doing nothing.”