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.
A WhatsApp spokeswoman said the company was “horrified at the attack” and was co-operating with the investigation.
Meanwhile, a 12th arrest has been made by officers investigating the attack. The 30-year-man was detained in Birmingham on Sunday on suspicion of preparing terrorist acts.
All messages sent on WhatsApp have end-to-end encryption, meaning they are unreadable if intercepted by anyone, including law enforcement and WhatsApp itself.
So while Masood’s phone is believed to have connected with the app, police may not know what, if anything, was communicated.
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.”
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.
The Facebook-owned company, which has a billion users worldwide, has said previously that protecting private communication is 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 the BBC’s Sunday Politics.
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