Amber Rudd has vowed to “call time” on internet firms who give terrorists “a place to hide” as it emerged security services are powerless to access Westminster attacker Adrian Ajao’s final WhatsApp message.

The Home Secretary said it was “completely unacceptable” that Whats App – which is owned by Facebook – was enabling terrorists to communicate “in secret”, knowing the police and security services will not be able to read their encrypted communications.

She has summoned WhatsApp, Facebook, Google and a host of other online firms to showdown talks at the Home Office on Thursday, where she says she will “call time” on extremists “using social media as their platform”.