Theresa May will make a plea to world leaders to combat the spread of online terror a the G7 summit
At a major international summit in Italy, the Prime Minister will warn that jihadi fanatics are taking their war against Western nation online to propagate their hate-filled ideology.
She will say leaders must unite to put pressure on internet giants such as Google, Twitter and Facebook to meet their "moral responsiblity" in rooting out extremism on social media.
And she will say social media firms must hand over online material that could identify extremists or face being "held to account".
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She will warn that jihadi fanatics are taking their war against Western nation online
Her stark warning, just days after the Manchester suicide bombing, will come in an address to US President Donald Trump and other leaders attending the gathering in Taormina, Sicily, of the G7 group representing some of the most economically powerful nations in the world.
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The Prime Minister will tell the G7: "The threat we face is evolving rather than disappearing as Daesh loses ground in Iraq and Syria.
The threat we face is evolving rather than disappearing as Daesh loses ground in Iraq and Syria
"The fight is moving from the battlefield to the internet."
In the aftermath of the Manchester attack, the use of social media to groom potential terrorist and instruct them in bomb making and other murderous techniques has been highlighted.
Ministers believe social media firms are still not doing enough to remove potentially harmful internet content or inform security services about suspected extremists.
The Prime Minister will say leaders must unite to put pressure on internet giants
At the G7, Mrs May will say Western governments need to work together on a "common approach" that will bring social media firms to heel.
She believes only united international action can "defeat Daesh online", according to senior Government officials.
Mrs May will seek to recruit G7 nations to adopt "a collective approach to working with tech companies on this agenda", a senior Government source said.
The Prime Minister is understood to want to bring pressure on the internet industry to improve self-regulation.
If they fail to co-operate, government should work together on legislation that would force them to act.
She will say the internet industry has a "social responsibility" to do more to remove harmful content from its networks.
Social media companies should be encouraged to develop computer technology and staffing procedures that can automatically identify and remove harmful material based on content and the identity the person posting it.
Officials say companies are already taking some steps to develop such an approach but their measures so far are inadequate given the vast scale of the extremist content online.
"We want to see companies block users who post extremist content and, where there is evidence, to report that to the relevent authorities," the senior Government source said.
Mrs May is to say that Islamic State fanatics are intensifying their use of online propaganda
Internet firms should also tell the authorities when they identify harmful material so action can be taken, the source said.
Mrs May also believes firms should revise conditions and industry guidelines to be "absolutely clear about what constitutes harmful material". Complanies that fail to cooperate with authorities "should be held to account", the source said.
Officials said Mrs May hoped the industry would respond as necessary without the need for legislation or measures such as fines.
"At the moment, we are setting out what we want companies to do. Let's wait and see what happens," the source said.