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.
In a speech in London as general election campaigning resumes after the attack, Mr Corbyn will point to links between wars abroad and “terrorism here at home”.
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.”
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.
BBC deputy political editor John Pienaar said Mr Corbyn’s speech about UK foreign policy, questions about police cuts and criticism of the government’s counter-extremism Prevent strategy were all likely to feed into the election debate as the truce draws to an end.
Speaking on the BBC’s Question Time on Thursday, Home Secretary Amber Rudd rejected Labour criticisms of police budget cuts and said: “We must not imply that this terrorist activity would not have taken place if there were more police.
“It’s not about those pure numbers on the street. It’s not about policing, so much as engaging with community leaders in the area.”
Mr Corbyn will pledge a “change at home and change abroad” if Labour wins power.
He will say that “many experts… have pointed to the connections between wars our government has supported or fought in other countries and terrorism here at home”.
“That assessment in no way reduces the guilt of those who attack our children. Those terrorists will forever be reviled and held to account for their actions.
“But an informed understanding of the causes of terrorism is an essential part of an effective response that will protect the security of our people that fights rather than fuels terrorism.”
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