David Cameron has threatened to legislate against internet search giants to force them to do more against online child pornography.
The prime minister is handing firms like Google, Bing, Yahoo! and others a deadline of October to act, amid frustration in Downing Street at the slow pace of progress by search engines in taking steps against child porn.
Cameron wants them to refuse to return results for searches for obviously obscene terms, for example.
"They're not doing nothing but we need them to do more," he told BBC1's The Andrew Marr Show.
"I believe in free speech but free speech doesn't mean you have the right to incite murder, to incite child abuse.
"There's a triangle here: the people uploading the images, the people looking at the images. There are also the companies enabling it to happen."
No 10 wants more action to get what Cameron called "vile images of child abuse" off the internet.
In a speech tomorrow the prime minister is set to outline a set of measures which will help - potentially including more resources for the Internet Watch Foundation and for police.
"You have a duty to act on this — and it is a moral duty," he is expected to tell the internet search engines.
"I have called for a progress report in Downing Street in October, with the search engines coming to update me.
"If in October... progress is slow or non-existent... then I can tell you that we are already looking at the legislative options we have to force action."
Some search engines have already introduced a 'splash page' warning computer users if they are close to viewing illegal images.
No 10 wants the messages to be stated in stronger terms, warning of a potential loss of job or even access to their own children if they continue.
A series of high-profile murder cases have fuelled the drive for reform.
Mark Bridger, who murdered five-year-old April Jones in Powys in October 2012, had violent pornographic images of children on his laptop.
Stuart Hazell, the killer of 12-year-old Tia Sharp, was found to have "extensive" pornography involving young girls when police searched his home.
Labour is calling for the government to do more - by banning the depiction of rape in extreme porn as other countries have done, for example.
"If David Cameron is really serious about protecting our children he should stop blocking compulsory sex and relationship education in our schools which could teach all our children zero tolerance of violence in relationships and greater resilience against online abuse," shadow home secretary Yvette Cooper said.
Not all are completely comfortable with the moves against online pornography, however.
Padraig Reidy of the Index on Censorship warned on the World At One programme that "a kind of default censorship" would be in place if an opt-in filter is introduced.
"Families should be able to choose if they want to opt in to censorship," he said.
"If a filter is set up as a default then it can really restrict what people can see legitimately.
"Sites about sexual health, about sexuality and so on, will get caught up in the same filters as pornography.
"It will really restrict people's experience on the web, including children's."