Leading search engine companies Google and Microsoft have agreed measures to make it harder to find child abuse images online.
As many as 100,000 search terms will now return no results that find illegal material, and will trigger warnings that child abuse imagery is illegal.
PM David Cameron has welcomed the move but said it must be delivered or he would bring forward new legislation.
Child protection experts have warned most images are on hidden networks.
In July, Mr Cameron called on Google and Microsoft's Bing - which together account for 95% of search traffic - to do more to prevent people getting access to illegal images.
He said they needed to ensure that searches which were unambiguously aimed at finding illegal images should return no results.
New software
Now both companies have introduced new algorithms [software instructions] that will prevent searches for child abuse imagery delivering results that could lead to such material.
Writing in the Daily Mail, Google executive chairman Eric Schmidt says: "These changes have cleaned up the results for over 100,000 queries that might be related to the sexual abuse of kids.
"As important, we will soon roll out these changes in more than 150 languages, so the impact will be truly global."
The restrictions will be launched in the UK first, before being expanded to 158 other languages in the next six months.
Alert warning
Warnings like this will appear on screen if users type in search terms related to child abuse
Mr Schmidt said warnings - from both Google and charities - would be shown at the top of search results for more than 13,000 queries, making it clear that child abuse was illegal.
Google communications director Peter Barron said: "We hope this will make a difference in the fight against this terrible crime."
Microsoft, which in a rare display of unity is working closely with Google on this issue, says its Bing search engine will also produce clean results.
The company said it had always had a zero tolerance attitude to child sexual abuse content and had been putting in place stronger processes to prevent access since the summer.
Later on Monday, the two companies will join other internet firms at Downing Street for an Internet Safety Summit.
The prime minister said significant progress has been made since his speech in July calling for action.
At the time, he said that Google and Microsoft had said that blocking search results couldn't be done but he did not accept that.
Tory MP Claire Perry, Mr Cameron's adviser on the sexualisation and commercialisation of childhood, told BBC Radio 4's Today programme the new measures were a "great step forward".
"We're not declaring victory but this is a massive step in the right direction," she said.
'Missed opportunity'
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This is the key child protection issue of a generation - we cannot fail”
Peter Wanless
NSPCC chief executive officer
But Jim Gamble, former head of the Child Exploitation and Online Protection Centre (Ceop) told BBC Breakfast he did not think the measures would 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," he said.
He said search engines had already been blocking inappropriate content and the latest move was just an enhancement of what was already happening.
A better solution would be to spend £1.5m on hiring 12 child protection experts and 12 co-ordinators in each of the police regions to hunt down online predators, he added.
NSPCC chief executive officer Peter Wanless said "a concerted and sustained effort from all quarters" was needed to stay one step ahead of sex offenders that were getting ever more technologically advanced.
"This is the key child protection issue of a generation - we cannot fail," he said.
Anxious child (model)
Critics have accused the government of underfunding online child protection
A June report by Ceop highlighted how the "hidden internet" helped distributors of child abuse images to evade detection by using encrypted networks and other secure methods.
Google and Microsoft have agreed to work with the UK's National Crime Agency and the Internet Watch Foundation to try to tackle networks which host child abuse images.
The two companies are also using their technological expertise to help in the identification of abuse images.
Microsoft's PhotoDNA already allows a photo to be given a unique "fingerprint" which means it can be tracked as it is shared across the internet. Now Google has developed VideoID which does the same job with videos.
Both firms will provide this technology to the National Crime Agency and other organisations to help in the work of finding and detecting those behind the creating and sharing of child abuse images.
But critics have accused the government of underfunding online child protection.
Ceop, which is now part of the National Crime Agency, has been accused of missing a recent opportunity to identify hundreds of people downloading illegal images.
Police in Toronto revealed that in 2012 they had shared hundreds of names of British people with Ceop who were alleged to be customers of a Canadian firm that sold videos of young children.
The operation to close down the business saw hundreds of people arrested in Canada and around the world - but none in Britain.
On Friday, the National Crime Agency said Ceop had examined the material but it had been classified as being on a low level of seriousness. However, the agency has now ordered a review of Ceop's handling of the case.
