UK Prime Minister Theresa May will meet French President Emmanuel Macron in Paris later for talks about anti-terror measures.
The two leaders have agreed proposals under which social media companies could be fined if they fail to remove extremist material.
It is Mrs May’s first foreign trip since losing her majority in a snap general election.
It comes as UK officials gear up for the start of Brexit talks on 19 June.
Mrs May and Mr Macron will have a working dinner and then attend the England v France football friendly at the Stade de France, where there will be a minute’s silence before kick-off to honour those killed in the Manchester and London attacks.
Speaking ahead of the visit, Mrs May said the two leaders were determined that the internet could not be used as a “safe space” for terrorists.
They are expected to agree ways that France and the UK can develop tools with tech companies such as Google and Facebook to stop online extremism.
The Metropolitan Police’s head of counter-terrorism Mark Rowley echoed the prime minister’s concerns that terrorist material was too easily accessible online.
Writing in the Times, Assistant Commissioner Rowley said: “We need communities to be more assertive at calling out extremists and radicalisers amongst us. It’s not just overseas propaganda inspiring attacks.
“And we need communications and internet-based companies to show more responsibility.
“It is too easy for the angry, violent or vulnerable to access extremist views, learn about attack methodologies, conspire on encrypted applications and then acquire equipment to kill, all online.”
Mr Rowley said “an internet going darker” was making it harder to look into people who may be of concern, but he welcomed Theresa May’s efforts to look at strategies for dealing with extremism.
The government’s independent reviewer of terrorism legislation, Max Hill QC questioned whether heavy fines for tech companies that failed to take down extreme content was “absolutely necessary”.
He told BBC News: “I’ve sat with the relevant police unit as they identify extreme content. I’ve seen them communicating with tech companies and I’ve seen the cooperation that flows from that.
“It’s a question of the bulk of the material rather than a lack of cooperation in dealing with it “
Google says it already invests heavily in combating abuse on its platforms and is working on an “international forum to accelerate and strengthen our existing work in this area”.
Facebook has also insisted it works “aggressively to remove terrorist content from our platform as soon as we become aware of it”.
Twitter says “terrorist content has no place on” its platform.