Every household in the UK is to have pornography blocked by their internet provider unless they choose to receive it, David Cameron is to announce.
In addition, the prime minister will say possessing online pornography depicting rape will be illegal.
In a speech, Mr Cameron will warn that access to online pornography is "corroding childhood".
Search engines will be given until October to introduce further measures to block illegal content.
In addition, experts from the Child Exploitation and Online Protection Centre (Ceop) will be given enhanced powers to examine secretive file-sharing networks, and 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.
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Under new measures, family-friendly filters will be automatically selected for all new internet customers - though they can choose to switch them off.
And millions of existing computer users will be contacted by their internet providers and told they must decide whether to activate filters to prevent their children accessing unsuitable material.
'Danger to children'
Mr Cameron will say: "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.
"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."
Mr Cameron will say that possession of pornography which is so extreme that it cannot even be bought in a licensed sex shop will be made illegal, bringing England and Wales in line with Scotland.
"These images normalise sexual violence against women - and they are quite simply poisonous to the young people who see them," he will say.
"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.
-BBC
