Britain is already in the "top tier" of targets for so-called Islamic State militants, David Cameron has told MPs.
He argued that military intervention was necessary to keep the country safe, and said an attack was likely even if Britain opted out of airstrikes.
"The House should be under no illusion - these terrorists are plotting to kill us, and to radicalise our children right now. They attack us because of who we are, not because of what we do," he said.
The shadow chancellor warned it could foster another generation of militants determined to commit a Paris-style terror attack.
The Defence Secretary has warned the RAF that the fight against Islamic State will not be short or simple.
Officers were posted outside the constituency office of Neil Coyle, the Labour MP for Bermondsey & Old Southwark.