Search engine giants Microsoft and Google will block Internet searches for child abuse images after months of mounting pressure.
The companies had initially insisted that it "couldn't be done, shouldn't be done", but the restrictions will now be launched in the UK first before being expanded to other English-speaking countries and 158 other languages in the next six months.
Prime Minister David Cameron hailed the decision by the two firms as "significant progress" after Google chief executive Eric Schmidt told the Daily Mail new software is to be introduced that will automatically block 100,000 "unambiguous" search terms which lead to illegal content.
A further 13,000 search terms linked with child sex abuse will flash up with warnings from Google and charities warning the user that the content could be illegal and pointing them towards help.
Cameron told the newspaper that child protection experts drew up the list of unique search terms which would undoubtedly lead to sex abuse images and videos.
"If you used these you were looking for child abuse images online," he said. “At the time, Google and Microsoft, who cover 95 per cent of the market, said blocking search results couldn't be done, that it shouldn't be done.
"They argued that it was against the very principle of the Internet and search engines to block material, even if there was no doubt that some of the search terms being used by paedophiles were abhorrent in a modern society. I did not accept that then and I do not accept that now."
Schmidt said Google has been working with Microsoft, which owns the Bing search engine, and law enforcement agencies since the summer following strong warnings from the Government to take action.
"We've listened, and in the last three months put more than 200 people to work developing new, state-of-the-art technology to tackle the problem," he said. "We've fine-tuned Google Search to prevent links to child sexual abuse material from appearing in our results.
"While no algorithm is perfect and Google cannot prevent paedophiles adding new images to the web, these changes have cleared up the results for over 100,000 queries that might be related to the sexual abuse of kids."
Google's new technology will also be able to remove up to thousands of copies of an illegal video in one hit by attaching a unique code to it which can remove all copies from the web.
The system is also designed to identify new code words or terms paedophiles start to use and can block search results for these too.
Former Child Exploitation and Online Protection Centre chief executive Jim Gamble said while it was "fantastic" that Cameron was involved, he was not sure how effective the initiative would be.
Gamble said search engines had already been blocking inappropriate content and the latest move was just an enhancement of what was already happening and the former senior policeman said Cameron's involvement would be better used by investing in child protection.
"I don't think this will make any difference with regard to protecting children from paedophiles," Gamble told BBC's Breakfast programme. "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."
Dr Martyn Thomas, chair of the Institution of Engineering and Technology IT policy panel, agreed.
“Universal blocking of websites, search terms and content is a blunt and ineffective tool and can easily be circumvented. The serious offenders are already using encryption and other technical means to hide their activities, which blocking by ISPs will not affect," he said.
“The Internet was designed to withstand serious damage and it treats censorship as damage and provides routes around it. There is no quick technical fix that will protect victims; it needs education, responsible parenting and more resources for enforcing the laws that already exist.”
The measures have been revealed by Google and Microsoft ahead of a cyber-summit on online child abuse at Downing Street today, but Cameron said he will work with the National Crime Agency to monitor the effectiveness of the new technology and is prepared to resort to law if it fails to work.
Cameron will also use the summit to unveil plans for Britain's National Crime Agency to join America's FBI in a new transatlantic taskforce to target paedophiles that use encrypted networks online.
No 10 said that the UK-US taskforce is being established by the US assistant attorney general and will be specifically tasked with tracking down offenders who use the "dark web" – secret and encrypted networks that are increasingly being exploited by paedophiles and other criminals.
The 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.
While some will be using them for legitimate purposes, UK law enforcement and intelligence agencies believe that paedophiles involved in distributing child abuse material are using them to hide their identities.
A Downing Street spokesman said: "Child abuse online is not restricted by international borders and so neither must our response. Parents on both sides of the Atlantic rightly expect us to take action on this and that is why we have reached out to secure a much closer working relationship with our American partners.
"This agreement will only strengthen our ability to crack down on this type of sickening crime."
At the same time a group of industry experts is being set up to look at new technical solutions for removing child abuse material from the Internet.
Joanna Shields, the chief executive of Tech City UK, said it would be looking to spot the "threats of future" to protect the most vulnerable in society.
"It's vital that governments and industry work together to eradicate child abuse content from the Internet, and that we mobilise the best and brightest in the technology industry to come up with innovative solutions to tackling this problem," she said.