Theresa May will urge world leaders to do more to combat online extremism, saying the fight against so-called Islamic State is “moving from the battlefield to the internet”.
Speaking about counter-terrorism at the G7 summit in Sicily, the PM will say more pressure should be put on tech companies to remove extreme material.
They should report such content to the authorities, she believes.
Labour leader Jeremy Corbyn is to say the “war on terror” is not working.
Twenty-two people were killed and 116 injured when a suicide bomber targeted an Ariana Grande concert in Manchester Arena on Monday evening.
Mrs May is expected to focus on online extremism when she chairs a counter-terrorism session at the summit in Italy.
Speaking to reporters outside Downing Street, she said she would lead a discussion on how to “work together to prevent the plotting of terrorist attacks online and to stop the spread of hateful extremist ideology on social media”.
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Mrs May added that co-operation from G7 and Nato would “enable us to work more closely together as we work to defeat the evil of terrorism”.
As Islamic State militants lose ground, the threat is “evolving rather than disappearing”, she will say, adding that the industry has a “social responsibility” to do more to take down harmful content.
She will acknowledge the industry has been taking action to remove extremist content, but will say it has not gone far enough and needs to do more.
And she will call for an international forum to develop the means of intervening where danger is detected, and for companies to develop tools which automatically identify and remove harmful material based on what it contains and who posted it.
French President Emmanuel Macron vowed France’s total support for Britain’s fight against terrorism as he met Mrs May at the summit.
“We will be here to cooperate and do everything we can in order to increase this cooperation at the European level, in order to do more from a bilateral point of view against terrorism,” he told her, in their first formal meeting since he took office.
Security minister Ben Wallace told the Today programme the use of online communications was “one of the biggest challenges” in the fight against terrorism, with encryption making it “almost impossible for us to actually lift the lid on these people”.
“And the scale of it is not just the UK, it is across the whole of Europe, across the world.”
He said the giant American tech companies like Facebook and Google could be doing more.
“We are determined to not let these people off the hook with the responsibility they have in broadcasting some horrendous [material], not only manuals about how to make bombs, but also grooming materials,” he said.
“We all think they could all do more… we need to have the tools to make them, where we need to, remove material quicker.”
Google said it was committed to creating an international forum designed to tackle extreme content online, to make sure “terrorists do not have a voice online”.
“We employ thousands of people and invest hundreds of millions of pounds to fight abuse on our platforms, and will continue investing and adapting to ensure we are part of the solution to addressing these challenges,” it added.
Meanwhile, Labour have attacked the government on police numbers, the prison service and foreign policy.
Mr Corbyn promised a “change at home and change abroad” if Labour wins power.
He says the UK “cannot be protected and cared for on the cheap” , and that “the war on terror is not working, we need a smarter way to reduce the threat from countries that nurture terrorists and generate terrorism”.
Theresa May: Online extremism ‘must be tackled’