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.
Well? Is checking on messages an invasion of people's privacy or a protection against terrorism? We can't have it both ways. We cannot limit our checks to 'suspects' if we can't check their communications to find out if they are suspects or not.