They challenged the security services’ recurrent dream in which they will gain the uncontested legal power to monitor all forms of communication – phone calls, email and social media – to discover who talks to whom, and when and where, even if they can’t find out what is said.

Parker said. “It’s important that MI5 should work within a transparent legal framework”.

Mr Parker dismissed the idea that the way MI5 worked was radicalising people, or that its strategy was failing.

Mr Parker, who played down worries about Islamist militants posing as refugees to enter Europe, said internet companies had a responsibility to share information about potential threats.

But he still gave a grim assessment of the situation facing the United Kingdom as he outlined the scale of the terror threat being tackled by his and the country’s other security agencies. To question it is not to underestimate the security threat that the United Kingdom faces.

“That is the highest number I can recall in my 32-year career, certainly the highest number since 9/11”, he said. It said, for example, that analysing communications data was crucial in identifying a group in the final stages of planning to blow up the London Stock Exchange and other locations in 2010. It’s a fundamental point about what MI5 is.

Home Secretary Theresa May is working on new legislation that would grant intelligence services greater powers in what is widely regarded as a revival of the controversial “Snooper’s Charter” that was rejected by the Conservative’s former coalition partners, the Liberal Democrats.

But Mr Parker said what should be included in new legislation was a matter “for parliament to decide”. And yet the MI5 statement declares: “Checking through large amounts of data is very often the only, and most crucial means MI5 has to track down terrorists who are plotting to cause harm to the UK”.

Acknowledging the fears surrounding government surveillance Parker clarified his organisation’s position saying: “We’re not about browsing through the private lives of the citizens of this country”.

The Prime Minister David Cameron himself sparked heavy debate over the issue when he suggested that a ban on encrypted services like WhatsApp and Snapchat would not be ruled out.

According to the head of the UK’s MI5 intelligence service, technology and the lack of cooperation from some tech firms is giving terrorists a big leg up on the government.

In response to a parliamentary question published Wednesday, Fallon said: “This figure is highly approximate, not least given the absence of United Kingdom ground troops in a position to observe the effects of strike activity”. Modern spies obviously feel they need to keep a close eye on GeoCities and MySpace.