The head of MI5 has said that the Prism data leaks scandal has damaged the UK's efforts to counter terrorism.
Speaking at the Royal United Services Institute in London last night, director general of MI5 Andrew Parker, said the internet and "leaps in technology" give opportunities to terrorists.
He said: "Through email, IP telephony, in-game communication, social networking, chat rooms, anonymising services, and a myriad of mobile apps, the terrorist has tens of thousands of means of communication, and many of these routes are now encrypted.
"Retaining the capability to access such information is intrinsic to MI5's ability to protect the country."
Parker said: "Let me be clear - we only apply intrusive tools and capabilities against terrorists and others threatening national security. The law requires that we only collect and access information that we really need to perform our functions, in this case tackling the threat of terrorism.
"In some quarters there seems to be a vague notion that we monitor everyone and all their communications, browsing at will through people's private lives for anything that looks interesting. That is, of course, utter nonsense."
Parker said that what MI5 knows about the terrorists and the secret details of the capabilities it uses against them represents its margin of advantage. That margin he said allows MI5 to detect their plots and stop them.
"But that margin is under attack," he said, implying that the Prism scandal, which has shown a light on MI5 and MI6 spying methods, is eroding it. "GCHQ [the secret service national listening centre] intelligence has played a vital role in stopping many of the terrorist plots that MI5 and the police have tackled in the past decade.
"It causes enormous damage to make public the reach and limits of GCHQ techniques. Such information hands the advantage to the terrorists. It is the gift they need to evade us and strike at will. Unfashionable as it might seem, that is why we must keep secrets secret, and why not doing so causes such harm."