The terrorist attacks in Paris, the thwarting of the terror plot in Belgium last week, and the heightened concern about the risk of attacks on Jewish targets across Europe, has left us all in a jittery mood.

Combined with the knowledge that several terrorist plots have already been foiled in the UK (at least four in 2014, according to the Metropolitan Police), and regular warnings from MI5 that another terrorist attack is highly likely, it can leave us all feeling pretty fretful about our safety and security. We also, quite naturally, look to the security services and politicians to do what they can to prevent an attack from happening.

But do the security services need more powers to protect us? The Home Secretary Theresa May thought so two years ago when she proposed a change to the law that would require internet service providers to maintain records of every user's activity. At the time, the Communications Data Bill (or the Snoopers' Charter as some called it) was blocked by the Liberal Democrats, but in the wake of the Paris attacks, the idea has been revived by the Prime Minister David Cameron.

In response, the leader of the Lib Dems Nick Clegg reiterated his opposition to the measure at the weekend, calling it unproven, clunky, and resource-intensive. But support for reform has now come from an authoritative source: the former MI5 chief Jonathan Evans.

Mr Evans, who was director-general of MI5 from 2007 to 2013, said yesterday that Britain's ability to prevent terrorist attacks was being hampered by outdated laws that were no longer fit for purpose. He said the legal powers under which the police and security agencies currently access communications for intelligence purposes have become outdated in the digital world and singled out Facebook, WhatsApp and Snapchat as particularly hard to monitor. "Technological changes," he said, "mean that it is much harder than it was a decade ago for the police or security agencies to find out what terrorists or criminals are saying among themselves."

Mr Evans is certainly right in raising the impact digital technology has had on terrorism. In the last few years, Islamist terrorists have changed the way they operate, demonstrating a combination of medieval fundamentalism and the most sophisticated digital awareness; they have used the internet to spread their ideology, recruit new followers and plan their attacks.

It is the security services' job to monitor this activity as best they can, but Mr Evans' case for more powers is far from proven. The security services do have a hard job to do, and, if there were an attack in Britain tomorrow, they would almost certainly be accused of not doing enough to prevent it. But the fact that digital technology has made it harder to monitor terrorist activity cannot lead us automatically to the assumption that every communication of every British citizen should be monitored all the time.

The Communications Data Bill would allow such surveillance, but there are ideological and practical reasons to question it. At the ideological level, can it ever be an appropriate response to the kind of attack on freedom we saw in Paris to further restrict freedom? And on the practical level, could blanket surveillance ever be made to work as effectively as targeted, evidence-led intelligence?

The security services should have the resources they reasonably need to do their job. But even in the face of the current threats to our safety, the starting point for any proposal for more powers should be a presumption against surveillance and in favour of our treasured right to privacy.