GCHQ's know-how will be used to "go after" paedophiles that exploit hidden parts of the internet to share vile images of child abuse, David Cameron has said.
The Prime Minister hailed a crackdown by Microsoft and Google on internet searches for horrific photographs and videos as "real progress against the absolute evil of child abuse" but said tackling the "dark net" was the next stage in the battle against online paedophilia.
New software is to being introduced that will automatically block 100,000 "unambiguous" search terms which led to illegal content on the two search engines but c hild protection campaigners warned that today's reforms fail to tackle the "dark corners of the internet" where paedophiles operate.
Speaking after a Downing Street summit with internet companies, Mr Cameron said he was "confident" that real progress could be made by using the "best brains" to track them down.
"There has been a lot in the news recently about the techniques, ability, and brilliance of the people involved in the intelligence community and GCHQ and the NSA in America," he told BBC Radio 2's Jeremy Vine show.
"That expertise is going to be brought to bear to go after on these revolting people sharing these images on the dark net and making them available more widely."
Britain's National Crime Agency is joining forces with America's FBI in a new transatlantic taskforce to target paedophiles who use encrypted networks online.
Mr Cameron added: " This is a day when we can make some real progress against the absolute evil of child abuse and what happens on the internet."
"The next stage is now to go after the dark net where people are sharing images peer to peer away from the Googles and the Microsofts. Again there is a lot more we can do, working with the industry, making sure we have got law enforcement which have got all the modern technological tools at their disposal and we will go after these people and arrest them and bang them up as well.
"The point is that you use technology which is able to get into the dark internet, which is able decrypt encrypted files and that is able to find out what is going on.
"Like all these things, if you put in the resources and the effort, if you use the best brains, the brains that are, as it were, the inheritors to the people that decrypted the enigma code in the Second World War, if you take those brains and apply it to the problem of tackling child abuse online you will get results."
No 10 said that the UK-US taskforce is being established by the US assistant attorney general and the British to target criminals who use the internet to hide from the law.
It will bring together the newly formed National Crime Agency and its American counterpart, the FBI, as well as other US law enforcement agencies. T he NCA estimates that the number of UK daily users of secret or encrypted networks will have risen to 20,000 by the end of the year.
Meanwhile, i nternet service providers will make family-friendly filters, which stop internet users viewing legal porn unless bill payers actively chose to turn them off, the default setting in most homes by the end of next year, No 10 said.
Mr Cameron insisted that the government was "not lecturing" parents but said that the move would mean couples would " have to have a discussion".
Martyn Thomas, who chairs the Institution of Engineering and Technology (IET) IT Policy Panel, said: "Whilst these moves to make it harder to find child abuse images online are a step in the right direction, they do not go far enough. There are better and more effective ways to protect children.
"The measures will help to protect young children from accessing such material, but they will do little for the people sharing these images which is being done through private peer-to-peer networks."
Former Child Exploitation and Online Protection Centre (CEOP) chief executive Jim Gamble told BBC's Breakfast programme: "I don't think this will make any difference with regard to protecting children from paedophiles.
"They don't go on to Google to search for images.
"They go on to the dark corners of the internet on peer-to-peer websites."