Revelations of the sort made by whistleblower Edward Snowden to The Guardian help terrorists evade capture and "strike at will" MI5 head Andrew Parker warned last night.
Speaking out about the leak of details about GCHQ surveillance methods, Parker said: "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."
The Daily Mail went further today and quoted security sources who said the Guardian revelations amounted to a "handbook" for terrorists.
— alan rusbridger (@arusbridger) October 8, 2013
Since June The Guardan has published many stories based on a cache of secret documents released to the paper by former US National Security Agency contracter Edward Snowden. It has used them to make extensive revelations about the surveillance and spying operations of US and UK security services.
Addressing the Royal United Services Institute, Parker, who took up the top role at MI5 in April, said: "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."
He said: "The converse to this would be to accept that terrorists should have means of communication that they can be confident are beyond the sight of MI5 or GCHQ acting with proper legal warrant," he said. "Does anyone actually believe that?"
Parker said understanding terrorists' capabilities gives The Security Service a "margin of advantage" – but warned that margin is "under attack" amid criticism over the reaches of GCHQ and other intelligence agencies.
He said: "Reporting from GCHQ is vital to the safety of this country and its citizens.
"GCHQ intelligence has played a vital role in stopping many of the terrorist plots that MI5 and the police have tackled in the past decade."
Defending the service's role, he added: "Far from being gratuitous harvesters of private information, in practice we focus our work very carefully and tightly against those who intend harm."
Parker also warned that can be "no such guarantee" that attacks can be prevented and added the service is "not perfect".
Speaking more broadly, the Director General said the service is "tackling threats on more fronts than ever before" and its task is "getting harder".
He said: "The threats are more diverse and diffuse. And we face increasing challenges caused by the speed of technological change."
Parker said he continues to expect at least one or two serious attempts at major acts of terrorism in the UK each year and it remains the case that there are several thousand Islamist extremists in the country who see British people as a "legitimate target".
A New Yorker profile of Guardian editor Alan Rusbridger revealed last month that Cabinet Secretary Jeremy Heywood had personally urged him not to publish the Snowden stories.
He reportedly told him: "By publishing this, you’re jeopardising not only national security but our ability to catch paedophiles, drug dealers, child sex rings. You’re an editor, but you have a responsibility as a citizen as well.”
A Guardian spokesman said: "A huge number of people from President Obama to the US Director of National Intelligence James Clapper have now conceded that the Snowden revelations have prompted a debate which was both necessary and overdue.
"The President has even set up a review panel and there have been vigorous discussions in the US Congress and throughout Europe. Such a debate is only worthwhile if it is informed. That is what journalism should do."