Online pornography to be blocked automatically in UK, says Cameron

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.

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.

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.

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 stress.

"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 Ceop's funding, will insist that the centre's experts and police will 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 will say.

Search engines, including Google, have a "moral duty" to block illegal content, Mr Cameron will say.

He will also call for warning pages to pop up when people try to search for illegal content.

On Sunday Mr Cameron called on internet companies to block access to material depicting child abuse.

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."

Civil liberties campaigners fear that blocking certain searches in one country could set a precedent elsewhere, making other governments more confident in applying censorship, said BBC technology correspondent Rory Cellan-Jones.