Encrypted messaging services such as WhatsApp should build back doors into their systems so intelligence agencies can read suspected terrorists' messages during investigations, the British government has demanded.
UK Home Secretary Amber Rudd said she supported end-to-end encryption, offered by the likes of WhatsApp, but insisted that security services must be able to eavesdrop on messages when they have a warrant.
It comes amid reports that terrorist Khalid Masood used WhatsApp seconds before launching Wednesday's attack at Westminster Bridge - but agencies are unable to see what was in his communication.
Ms Rudd also insisted that the likes of Google, which runs the social video sharing platform YouTube and smaller sites such as WordPress, must realise that they were now publishing - rather than technology - companies and should take more responsibility for taking down extreme material.
The home secretary left the door open to changing the law if necessary.
But she said she would rather see an industry-wide board doing it independently, as the best people to take action are those who understand the technology and the "necessary hashtags".
On encrypted messaging services, she told the BBC: "It is unacceptable, there should be no place for terrorists to hide.
"We need to make sure that organisations like WhatsApp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other."
In the US, Apple fought the FBI's request for the passcodes needed to unlock an iPhone that had been used by one of the perpetrators in the 2015 extremist attack in San Bernardino, California. The FBI eventually found another way around the lock.
"Hero" MP Tobias Ellwood has said he is "heartbroken" that his efforts to save the life of a police officer during the Westminster terror attack were ultimately unsuccessful.