Theresa May believes internet companies need to go "further and faster" in fighting terrorism online.
The Prime Minister will co-host an event on preventing terrorist use of the internet at the United Nations General Assembly in New York later today.
Ahead of the meeting, she said: "We need a fundamental shift in the scale and nature of our response - both from industry and governments - if we are to match the evolving nature of terrorists' use of the internet."
During the talks, Mrs May will say: "Industry needs to go further and faster in automating the detection and removal of terrorist content online, and developing technological solutions which prevent it being uploaded in the first place."
The event will also be attended by French President Emmanuel Macron and Italian Prime Minister Paolo Gentiloni.
Few are likely to disagree with Mrs May's remarks - including the technology giants themselves.
Twitter alone has suspended nearly 300,000 accounts so far this year, with the company revealing 95% of those were detected automatically.
A Number 10 source said: "They have been doing something, but just not enough. These companies have some of the best brains in the world.
"They should really be focusing that on what matters, which is stopping the spread of terrorism and violence. We want them to break the echo chambers."
Sky News has learned WhatsApp rejected a UK Government request to access encrypted messages this summer.
The inability to access terrorists' encrypted conversations is creating a "black hole" for security services, according to an intelligence source.
Terrorists are "frequent users of encrypted apps - specifically WhatsApp and Telegram", the source said.
"It is crucially important that we can access their communications and, when we can't, it can provide a black hole for investigators," the source added.
Encrypted messaging apps were used ahead of the terror attacks in Westminster, Manchester and London Bridge this year.
The devices of one of the Parsons Green suspects have been recovered and are being investigated by intelligence agencies.
Apps such as WhatsApp, Telegram and Apple (NasdaqGS: AAPL - news) iMessage employ end-to-end encryption, meaning messages sent by users are scrambled by a code contained on the app on their phone or the phone itself.
The systems are designed so that tech companies themselves do not even have access to the contents of a message.
Sky News understands that WhatsApp co-operates with law enforcement to provide the metadata it does hold - the name of an account, when it was created, the last seen date, the IP address and associated email address.
But the company argues it can't provide data is doesn't collect in the first place, including the contents of a message.
In a statement on its website, the company states: "Naturally, people have asked what end-to-end encryption means for the work of law enforcement.
"WhatsApp appreciates the work that law enforcement agencies do to keep people safe around the world.
"We carefully review, validate, and respond to law enforcement requests based on applicable law and policy, and we prioritise responses to emergency requests."
Raffaello Pantucci, director of international security studies at the Royal United Services Institute, told Sky News: "The way you have to think that the terrorist groups use these sorts of applications is the same way that all of us use encrypted applications, which is to plan our lives.
"So what you're increasingly seeing is that individuals are forming relationships or advancing relationships using these online communication methods.
"And they're using this to direct plots. We've had some instances where you've had individuals based in Raqqa, Syria, who are steering people to launch terrorist attacks from a distance and are communicating with them blow by blow using these encrypted communications."
Sky News understands 80% of investigations into terrorism and serious crime are now impacted by encryption.
Terrorists themselves are now "far more cyber savvy" than they have been before, the security source told Sky News.
At a meeting this summer, British officials asked WhatsApp to come up with technical solutions to allow them to access the content of messages - effectively, a back door.
Cybersecurity experts say creating a back door for a government would weaken encryption for everyone, as it would be a prime target for hackers.
But UK intelligence officials believe a compromise could be possible, pointing out cybersecurity isn't binary, and that services offer different levels of cybersecurity to deal with different levels of threats.
The UK Government remains firm in its aspiration to obtain the unencrypted content of messages, under a proper warrant.
The problem of obtaining access to the content of communications is larger than any one company.
Telegram remains popular for dissemination of jihadist materials.
Sky News observed channels - including those of al Qaeda's official mouthpiece and Abu Qatada, the radical cleric deported to Jordan - pumping out propaganda.
Recruiters pump out content to lure the curious.
That can inspire attacks - or they can go one further and use the same app to direct attacks on another continent.
Telegram told Sky News it has a dedicated channel to remove terrorist accounts.
"As a result, jihadi channels usually go down within a few hours, long before they can reach any traction," the company said.
Earlier this year, Facebook, YouTube, Twitter and Microsoft (Euronext: MSF.NX - news) , along with other tech companies, announced they would form the Global Internet Forum to Counter Terrorism in an attempt to tackle extremism on their platforms.