British Prime Minister David Cameron today will announce an aggressive plan to crack down on online pornography, as part of an ongoing campaign to protect children from its "corroding" influence. Under Cameron's plan, online porn will be blocked by default for all new UK household internet connections, meaning consumers will have to tell their internet service providers (ISPs) whether they want to disable the filters. The plan will also make it a crime to possess "extreme pornography," such as images or video of simulated rape, the Daily Mail reports.

"By the end of this year, when someone sets up a new broadband account the settings to install family-friendly filters will be automatically selected," Cameron will say in a speech to the child protection group NSPCC on Monday. "If you just click 'next' or 'enter', then the filters are automatically on."

"You have a duty to act on this — and it is a moral duty."

Cameron is taking aim at violent and illegal child pornography, in particular, and will call upon major search engines to do their part to prevent "sick" people from accessing such content. Under the initiative, the Child Exploitation and Online Protection Centre (CEOP) will draft a blacklist of objectionable search terms, while a UK-US joint task force will be established to help combat obscene websites.

"I have a very clear message for Google, Bing, Yahoo and the rest," Cameron will say. "You have a duty to act on this — and it is a moral duty. If there are technical obstacles to acting on [search engines], don't just stand by and say nothing can be done; use your great brains to help overcome them."

The prime minister will meet with leading search engines in October to discuss their progress in blocking all terms included on CEOP's blacklist. "If the progress is slow or non-existent," Cameron says, "then I can tell you we are already looking at the legislative options we have to force action."

Today's announcement has been expected for several weeks now, and was immediately welcomed by Rape Crisis, a London-based advocacy group. It is illegal to publish or possess rape pornography in Scotland, but has thus far remained legal in both England and Wales.

"challenging the eroticization of violence against women and girls"

"We are heartened by the government's announcement that it will close the loophole in existing extreme pornography legislation," Fiona Elvines of Rape Crisis South London told the Guardian. "The government today has made a significant step forward in preventing rapists using rape pornography to legitimize and strategize their crimes and, more broadly, in challenging the eroticization of violence against women and girls."

But others have raised doubts about whether Cameron's plan can actually work. The mandatory ISP filters have proven particularly contentious, as evidenced by a leaked letter from the Department of Education that the BBC published last week.

"To work, the filters would need to prevent users from asking search engines 'How do I turn off these porn filters?'" Tom Melzer wrote in a Sunday editorial for the Guardian, echoing ISP concerns that web filters could be easily circumvented. "And then the question 'How do I turn off the filters for questions about turning off filters?' And so on, forever."

It isn't immediately clear how Cameron's plan will affect existing internet subscribers, though the BBC's leaked letter suggests that for them, the filters will remain opt-in. UK ISPs currently offer an "Active Choice" program, whereby users can choose the degree to which online content is filtered.

"The prime minister believes that there is much more that we can all do to improve how we communicate the current position on parental internet controls and that there is a need for a simplified message to reassure parents and the public more generally," the letter reads. "Without changing what you will be offering (ie active-choice +), the prime minister would like to be able to refer to your solutions are 'default-on' as people will have to make a choice not to have the filters (by unticking the box)."

Cameron, meanwhile, seems intent on pushing his plan forward, framing the issue as a moral imperative.

"I'm not making this speech because I want to moralize or scaremonger, but because I feel profoundly as a politician, and as a father, that the time for action has come," he will say. "This is, quite simply, about how we protect our children and their innocence."

Update: In his speech, Cameron detailed how the initiative would be applied to existing customers. "Following the work we’ve already done with the service providers, they have now agreed to take a big step," Cameron said. "By the end of next year, they will have contacted all of their existing customers and presented them with an unavoidable decision about whether or not to install family friendly content filters." Claire Perry, Tory Minister of Parliament and Cameron's adviser on the matter, told the BBC that existing customers who do not inform ISPs of their decision will have the "family-friendly filters" implemented automatically.