David Cameron has told ITV News that the terrorist threat to the UK "is the greatest concern that I have as Prime Minister" as he renewed calls for the introduction of the so-called 'snooper's charter' to monitor extremists.
The Prime Minister was speaking after attending a unity rally in Paris in memory of the 17 victims of last week's terror attacks.
Plans for a communications data bill - branded a "snooper's charter" by critics - were blocked by Liberal Democrat opposition, but Mr Cameron has suggested he will revive the legislation if he wins May's general election.
"We do need to modernise our rules about interception," he told Chris Ship.
"I think we cannot allow modern forms of communication to be exempt from the ability, in extremism, with a warrant signed by the Home Secretary, to be exempt from being listened to," he said.
"That is my very clear view and if I am Prime Minister after the next election I will make sure we legislate accordingly."
Asked about his concerns that the UK could be the victim of an attack similar to the one carried out in Paris last week, Mr Cameron said: "It is the greatest concern I have as Prime Minister.
"We do face a very similar threat in Britain to the threat in France."
The Prime Minister said security officials had prevented "several" plots in recent months.
He also refused to speculate that the terror threat level could be raised from severe to critical in the wake of the attack on Paris.
His comments come just days after the head of MI5 warned that a group of al-Qaida terrorists in Syria is planning "mass casualty attacks" against the West.
Mr Cameron added: "Our security and intelligence services have done a brilliant job at finding out, intercepting and preventing plots including several in recent months - attempts to kill police officers for instance.
"You can never be certain that you will always be certain you will be able to stop every potential attack.
"We need to focus on strengthening our intelligence and security.
"But also crucially combating this appalling narrative, this death cult of Islamist extremism wherever we find."