Most households in the UK will 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.
Mr Cameron warned in a speech 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.
He told the BBC he expected a "row" with service providers who, he said in his speech, were "not doing enough to take responsibility" despite having a "moral duty" to do so.
He also warned he could have to "force action" by changing the law and that, if there were "technical obstacles", firms should use their "greatest brains" to overcome them.
'Innocence'
In his speech, Mr Cameron said family-friendly filters would be automatically selected for all new customers by the end of the year - although 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 use or not use "family-friendly filters" to restrict adult material.
The filters would apply to all devices linked to the affected home Wi-Fi network and across the public Wi-Fi network "wherever children are likely to be present".
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.
The UK's biggest internet service providers have agreed to the filters scheme meaning it should cover 95% of homes.
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.
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.
"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."
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.
"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.
SOURCE
Personally I think this is a good thing. Protecting children from online pornography is a good thing- younger and younger children are stumbling across adult content and getting warped ideas about sex and women. I'm not saying all porn is warped, or wrong, but there is some pretty hardcore stuff just a few clicks away from google. I think only adults should be able to decide if they personally want to watch pornography by opting in to the service.
Some people have said that children should instead be monitored on the computer, not be allowed their own laptops etc but it's physically impossible to monitor a child the whole time. Say you're a single parent, or your partner is at work, and you have the housework to do, dinner to cook, other children to watch and your kid wants to do their homework online? You can't sit beside them the whole time. Or if they go round their friends house and their parents don't care what they look at?
Obviously teenagers will be more savvy with technology and could use their mobile phones, ipads etc to watch porn but hopefully it won't be as detrimental to them as it will to children, and their parents will hopefully have talked to them about how unrealistic pornography is.
