The former head of MI5 has warned that Britain's anti-terror laws are "not fit for purpose".
Lord Evans' comments about the powers of intelligence and security agencies were reported as the Prime Minister said Britain is facing a "very severe threat" from Islamist extremists, including so-called "lone-wolf" terrorists.
Lord Evans told the Sunday Telegraph: "The ability of the police and security agencies to do this important work of protecting our society and its vulnerable people is under threat from changing technology.
"They can only do this if they have the tools to do so - and the tools at their disposal are no longer fit for purpose."
David Cameron is currently pushing for tougher requirements for internet firms to alert authorities to suspicious online exchanges, ban encrypted communications and store data.
Former Lib Dem leader Paddy Ashdown hit out at the push for increased powers, telling Sky's Murnaghan programme: "The whole nation is made a suspect under these proposals."
He added that while he believed Mr Cameron's demand for increased powers was "sincere", it was "connected to the general election", and said the PM was "cavalier" about the protection of civil liberties.
When asked if the message from the US to allies would be to keep up with the changes to anti-terror laws implemented by Washington, the US ambassador to the UK, Matthew Barzun, told the same programme: "Sure, and we work very closely with the UK and other partners around the world to try to achieve that balance."
Mr Cameron earlier spoke to US TV network CBS about measures to tackle extremism at home.
He said: "In Britain's history, we've had some very intense times of terrorist threats.
"Certainly we face a very severe threat.
"That's what we're calling it: severe, because we believe an attack is highly likely.
"But frankly, we've been in this struggle against extremist, Islamist terrorism now for well over a decade and a half, so we know what it takes to win, but it's going to take a lot of perseverance."
He said the terror threat "keeps morphing" over time, "but it's still based on the fundamental problem of a poisonous death cult narrative which is the perversion of one of the world's major religions".
Foreign Secretary Philip Hammond and his US counterpart John Kerry are set to host talks in London with ministers from around 20 countries, including Arab states, to discuss ways of tackling Islamic State.
Members of the coalition fighting IS extremists in Syria and Iraq will meet for the summit on Thursday.
On Friday Mr Cameron vowed to tackle what he called the "poisonous narrative of Islamist extremism that is turning too many young minds".
He was speaking following two days of talks with US President Barack Obama in Washington where they said a new group would be set up to exchange information and expertise to tackle the terror threat.
There are fears about the spread of terrorism after the Paris attacks that left 17 people dead and the arrest of more than two dozen people in anti-terror raids in Belgium, Germany and France.
On Saturday, an 18-year-old woman who was arrested at Stansted airport in Essex on suspicion of terrorist offences was released on bail.
Anti-terror officers arrested the teenager when she arrived on a flight at around 4pm on Friday.
