Every household in the UK is to have pornography blocked by their internet provider unless they choose to receive it, David Cameron has announced.
In addition, the prime minister said possessing online pornography depicting rape would become illegal in England and Wales - in line with Scotland.
In a speech, Mr Cameron warned that access to online pornography was "corroding childhood".
The new measures will apply to both existing and new customers.
Mr Cameron also called for some "horrific" internet search terms to be "blacklisted", meaning they would automatically bring up no results on websites such as Google or Bing.
But he told BBC Radio 4's Woman's Hour programme he expected a "row" with service providers, who argued that, as a matter of "freedom of speech", customers should be able to type in any words they wanted.
'Innocence'
In his speech, Mr Cameron said family-friendly filters would be automatically selected for all new customers - though they could choose to switch them off.
And millions of existing computer users would be contacted by their internet providers and told they must decide whether to activate "family-friendly filters" to restrict adult material.
Customers who do not click on either option - accepting or declining - will have filters activated by default, Tory MP Claire Perry, Mr Cameron's adviser on the sexualisation and commercialisation of childhood, told the BBC.
Other measures announced by the prime minister included:
New laws so videos streamed online in the UK will be subject to the same restrictions as those sold in shops
Search engines having until October to introduce further measures to block illegal content
Experts from the Child Exploitation and Online Protection Centre being given more powers to examine secretive file-sharing networks
A secure database of banned child porn images gathered by police across the country will be used to trace illegal content and the paedophiles viewing it
Mr Cameron also called for warning pages to pop up with helpline numbers when people try to search for illegal content.
He said: "I want to talk about the internet, the impact it is having on the innocence of our children, how online pornography is corroding childhood.
"And how, in the darkest corners of the internet, there are things going on that are a direct danger to our children, and that must be stamped out.
On Sunday, Mr Cameron called on internet companies to do more to block access to material depicting abuse
"I'm not making this speech because I want to moralise or scaremonger, but because I feel profoundly as a politician, and as a father, that the time for action has come. This is, quite simply, about how we protect our children and their innocence."
But former Child Exploitation and Online Protection Centre boss Jim Gamble told BBC Radio 4's Today programme it was important to "get to the root cause" of illegal pornography, by catching those responsible for creating it.
He added: "You need a real deterrent, not a pop-up that paedophiles will laugh at."
But Ms Perry argued filters would make a difference, saying that the killers of schoolgirls April Jones and Tia Sharp had accessed legal pornography before moving on to images of child abuse.
She added: "It's impossible to buy this material in a sex shop... but it's possible to have it served up on a computer every day."
In his speech, Mr Cameron said possession of online pornography depicting rape would be made illegal.
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The coalition government has pledged to prevent abuse of women and girls, so tackling a culture that glorifies abuse is critical for achieving thisâ€
Holly Dustin
Director, End Violence Against Women Coalition
Existing legislation only covers publication of pornographic portrayals of rape, as opposed to possession.
"Possession of such material is already an offence in Scotland but because of a loophole in the Criminal Justice and Immigration Act 2008, it is not an offence south of the border," Mr Cameron said.
"Well I can tell you today we are changing that. We are closing the loophole - making it a criminal offence to possess internet pornography that depicts rape."
The move has been welcomed by women's groups and academics who had campaigned to have "rape porn" banned.
Holly Dustin, director of the End Violence Against Women Coalition, said the group was "delighted".
"The coalition government has pledged to prevent abuse of women and girls, so tackling a culture that glorifies abuse is critical for achieving this," she said.
"The next step is working with experts to ensure careful drafting of the law and proper resourcing to ensure the law is enforced fully."
'No safe place'
Mr Cameron, who has faced criticism from Labour over cuts to Child Exploitation and Online Protection Centre's funding, insisted the centre's experts and police would be given the powers needed to keep pace with technological changes on the internet.
Claire Lilley, NSPCC: "In every single child abuse image there is a victim, a child who has been abused"
"Let me be clear to any offender who might think otherwise: there is no such thing as a safe place on the internet to access child abuse material," he said.
A spokesman for Google said: "We have a zero tolerance attitude to child sexual abuse imagery. Whenever we discover it, we respond quickly to remove and report it.
"We recently donated $5m (£3.3m) to help combat this problem and are committed to continuing the dialogue with the government on these issues."
According to some experts, "default on" can create a dangerous sense of complacency, says BBC technology correspondent Rory Cellan-Jones.
He says internet service providers would dispute Mr Cameron's interpretation of the new measures, insisting they did not want to be seen as censors.
http://www.bbc.co.uk/news/uk-23401076
Sucks to be in Great Britian I suppose. Looks like Great Britian is run by a bunch of prudes. This makes the US government look better but if things keep going as they are in the US, it won't be long till we start seeing this kind of thing.