A senior British lawmaker has blasted messaging platforms like WhatsApp for using end-to-end encryption, which stops security services monitoring messages.

It comes as it emerges the London terror attacker Khalid Masood used the messaging service moments before he killed an American tourist, a police officer and two others outside the U.K. Parliament.

British Home Secretary Amber Rudd told the BBC it was "completely unacceptable" for messaging services to encrypt communication.

"We need to make sure that organizations like WhatsApp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other," she said.

Rudd also urged technology firms to do a better job preventing the publication of material that promotes extremism.

Police have said they are currently investigating the London attacker's path to radicalization and whether he was in contact with other terrorists before the assault.

It follows an ongoing battle between technology giant Apple and the FBI in the U.S. over access to locked devices.

Read more:  NBC News