WhatsApp must not be 'place for terrorists to hide'
There must be "no place for terrorists to hide" and intelligence services must have access to encrypted messaging services, the home secretary has said.
Khalid Masood killed four people in Westminster this week. It is understood his phone had connected to messaging app WhatsApp two minutes earlier.
Amber Rudd said she would be meeting technology firms this week to ask them to "work with us".
Labour leader Jeremy Corbyn said authorities already had "huge powers".
There had to be a balance between the "right to know" and "the right to privacy", he said.
Speaking to BBC One's Andrew Marr Show, Ms Rudd said: "It is completely 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.
"It used to be that people would steam open envelopes or just listen in on phones when they wanted to find out what people were doing, legally, through warranty.
"But on this situation we need to make sure that our intelligence services have the ability to get into situations like encrypted WhatsApp."
All messages sent on WhatsApp have end-to-end encryption.
This means messages are unreadable if they are intercepted by anyone, including law enforcement and WhatsApp itself.
The Facebook-owned company, which has a billion users worldwide, has said protecting private communication was one of its "core beliefs".
Tim Cook, the chief executive of Apple which also uses end-to-end encryption, has previously said it would be "wrong" for governments to force Apple to "build a back door" into products.
But Ms Rudd said: "I would ask Tim Cook to think again about other ways of helping us work out how we can get into the situations like WhatsApp on the Apple phone."
Europol director Rob Wainwright echoed Ms Rudd's call for changes.
"I would agree something has to be done to make sure that we can apply a more consistent form of interception of communication in all parts of the way in which terrorists invade our lives," he told Andrew Neil on Sunday Politics.
Masood, 52, killed three people and injured 50 when he drove a car into pedestrians on Westminster Bridge on Wednesday.
He then fatally stabbed a police officer before being shot dead by police - all within 82 seconds.
Ms Rudd would not confirm who shot Masood, amid claims it was a bodyguard for Defence Secretary Sir Michael Fallon.
On Saturday the Metropolitan police said they believed Masood acted alone.
But they added they were also "determined" to find out whether he had been inspired by terrorist propaganda.
Scotland Yard said it was possible "we will never understand why he did this".
Ms Rudd called on social media companies to develop "technology solutions".
She said: "Each attack confirms again the role that the internet is playing in serving as a conduit, inciting and inspiring violence, and spreading extremist ideology of all kinds.
"We need the help of social media companies, the Googles, the Twitters, the Facebooks of this world.
"And the smaller ones, too: platforms such as Telegram, Wordpress and Justpaste.it."
But chairman of the home affairs select committee, Yvette Cooper, told Sky News's Sophy Ridge on Sunday it was "not enough" for the government to have more meetings with technology companies.
- 14:40:08 - the car that Masood was driving over Westminster Bridge first mounted the pavement on the northbound side
- 14:40:38 - after continuing towards Bridge Street along both the footpath and road, Masood crashes into the perimeter fence of the Palace of Westminster
- 14:40:59 - the first 999 call was made to the Met police reporting the incident
- 14:41:30 - Masood left the vehicle and was shot by a police firearms officer inside the Palace of Westminster
Writing in the Sunday Telegraph, the home secretary also said she was asking companies like Google, Twitter and Facebook to be more "proactive" in tackling extremism.
In the Sunday Times, Foreign Secretary Boris Johnson also called for internet companies to develop technology to detect and remove extreme material.
He said: "They are not acting when they are tipped off. Evil flourishes when good men do nothing - and that's what's happening here.
"They are putting up adverts next to it."
Earlier this month, Google's European boss apologised after adverts from major firms and government agencies appeared next to extremist content on its YouTube site.
Matthew Brittin promised to review the firm's policies and strengthen enforcement.
Marks and Spencer and Audi were among the companies that pulled their online adverts over the issue.