European Union leaders have threatened to pass legislation to force internet companies to take down extremist and terrorist material if they fail to act voluntarily.
The move came after Prime Minister Theresa May urged EU counterparts at a Brussels summit to step up pressure on the tech giants in the wake of attacks in London and Manchester.
Mrs May and French president Emmanuel Macron agreed in Paris last week to press social media and tech firms to move forward with the establishment of an industry-led forum to develop tools to automatically identify and remove unacceptable material, with the threat that regulatory or legislative action could follow if they fail to do so.
Announcing the EU's position in a Brussels press conference, European Council president Donald Tusk said: "Terrorism is still a major threat. We are fully determined to protect our people.
"To that end, the European Council agreed to deepen our efforts against foreign terrorist fighters."
He said that leaders of the 28 EU states had agreed on the need to "co-operate closely with the online industry".
"We are calling on social media companies to do whatever is necessary to prevent the spread of terrorist material on the internet.
"In practice, this means developing new tools to detect and remove such material automatically. If need be, we are ready to also adopt relevant legislation."
The move comes amid concerns that terrorists of the kind involved in attacks at Manchester and London may have been radicalised by material accessed via sites such as Facebook or YouTube and may have been provided with opportunities to communicate secretly using end-to-end encrypted systems such as WhatsApp.
Leading a discussion on counter terrorism at the European Council summit, Mrs May urged EU leaders to help to "rid terrorist material from the internet in all our languages".
Just as she and Mr Macron have agreed to use national expertise and resources to work with tech companies to tackle terrorist content on English and French-language internets, other EU leaders should do the same with sites in their own languages, she said.
Mrs May thanked other leaders for the sympathy and condolences offered following the recent spate of terror attacks in the UK.
She hailed EU nationals for their heroism, including Spanish bank worker Ignacio Echeverria, who died trying to save a woman from an attacker at London Bridge, and Romanian baker Florin Morariu, who fought off the terrorists and gave shelter to terrified Londoners.
Calling for "unity in the face of terror", the PM said it was only through co-operation and robust defence of shared values, that European nations will defeat the evil of terrorism.