Social media companies such as Facebook and Twitter could be fined if they fail to remove extremist propaganda and terrorist material under proposals agreed by Prime Minister Theresa May and French President Emmanuel Macron.
Speaking ahead of a visit to Mr Macron in Paris on Tuesday in the wake of a series of jihadi attacks in Westminster, Manchester and London Bridge, Mrs May said she and the president were determined to ensure the internet could not be used as a safe space for terrorists and criminals.
The UK and France are to develop plans to create a new legal liability for tech companies which fail to take action against unacceptable content on their platforms.
The two countries are to lead joint work with internet giants to explore the potential for new tools to identify and remove harmful material automatically.
Mrs May's visit comes just days after legislative elections in France which appear to have delivered Mr Macron's En Marche party an overwhelming dominance in parliament, just as the UK General Election deprived the Prime Minister of her own Commons majority.
After talks and a working dinner at the Elysee Palace, the pair are set to travel to the Stade de France to watch England take on France in a friendly football international.
Mrs May's official spokesman was not immediately able to confirm whether the Prime Minister, a keen cricket fan, had seen England play before.
Mrs May said: "The counter-terrorism co-operation between British and French intelligence agencies is already strong, but President Macron and I agree that more should be done to tackle the terrorist threat online.
"In the UK we are already working with social media companies to halt the spread of extremist material and poisonous propaganda that is warping young minds.
"And today I can announce that the UK and France will work together to encourage corporations to do more and abide by their social responsibility to step up their efforts to remove harmful content from their networks, including exploring the possibility of creating a new legal liability for tech companies if they fail to remove unacceptable content.
"We are united in our total condemnation of terrorism and our commitment to stamp out this evil."
Mrs May and Mr Macron will press tech companies to move forward urgently with the establishment of an industry-led forum to develop shared technical and policy solutions to the problem, as agreed by leaders of the world's most advanced economies at last month's G7 summit in Italy.
Home Secretary Amber Rudd and French interior minister Gerard Collomb will meet in the coming days to drive the agenda forward.
Labour MP Yvette Cooper, who chaired the House of Commons Home Affairs Committee in the last parliament, said: "Social media companies like YouTube have been getting away with a dangerous and irresponsible approach to extremism for too long.
"Still today YouTube is showing illegal propaganda videos for banned jihadi and neo-Nazi extremists. They have a disgraceful disregard for the law.
"The cross-party Home Affairs Select Committee called for a system of fines and stronger legislation.
"So if that is what the British and French governments are working on now, that is really welcome.
"They need to make rapid progress, because online radicalisation is a very serious threat, and this problem has been growing for a long time."