Exposure 'damages' security service
Exposing the "reach and limits" of listening post GCHQ causes enormous damage and hands the advantage to terrorists, the head of MI5 has warned in his first public speech.
Andrew Parker's comments come after the surveillance activities of GCHQ and its American counterpart the National Security Agency (NSA) were disclosed by the former US intelligence operative Edward Snowden.
The Director General dismissed the notion that The Security Service monitors "everyone and all their communications" as "utter nonsense".
Addressing the Royal United Services Institute (RUSI), Mr Parker, who took up the top role 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."
Mr Snowden, who is currently in Russia, leaked information to The Guardian in May that revealed mass surveillance programmes such as the NSA-run Prism and the GCHQ-operated Tempora.
Under the £1 billion Tempora operation, Cheltenham-based GCHQ is understood to have secretly accessed fibre-optic cables carrying huge amounts of internet and communications data and shared the information with the NSA.
Mr Parker, who replaced Sir Jonathan Evans, said MI5 needs to be able to read or listen to terrorists' communications in order to stop them.
"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?"
Mr 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."
He went on: "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."
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."
Mr Parker also warned that can be "no such guarantee" that attacks can be prevented and added the service is "not perfect".
The Director General said eliminating terrorism entirely is not possible in the face of "persistent and serious" threats.
A number of terrorist attacks and suspected terror-related incidents have occurred on British soil involving individuals reportedly already known to the intelligence agencies.
The Director General said: " Knowing of an individual does not equate to knowing everything about them. Being on our radar does not necessarily mean being under our microscope.
"The reality of intelligence work in practice is that we only focus the most intense intrusive attention on a small number of cases at any one time.
"The challenge therefore concerns making choices between multiple and competing demands to give us the best chance of being in the right place at the right time to prevent terrorism."
He added: " We are not perfect, and there are always things we can learn, do better and sharpen up on."
Mr Parker also reiterated his predecessor's warnings about Syria - as a large proportion of casework at MI5 now concerns individuals from the UK who have travelled to fight in the conflict-torn country.
"For the future, there is good reason to be concerned about Syria," he said.
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."
Mr Parker said he continues to expect at least one or two serious attempts at major acts of terrorism in the UK each year and i t remains the case that there are several thousand Islamist extremists in the country who see British people as a "legitimate target".
The Director General, who has been with The Security Service since the early 1980s, led the agency's response to the July 7 attacks on London in 2005 as director of international terrorism.
In 2006, his teams played the lead role in the disruption of al Qaida's attempt to attack multiple airliners with bombs hidden in drinks bottles.
Nick Pickles, director of civil liberties campaigners Big Brother Watch, said: "This speech is more striking for what it did not say.
"No recognition that Britain's legal framework was drawn up before Google existed, no acknowledgment that our antiquated oversight mechanism may need further reform, no mention of the US Government's moves towards increased transparency and oversight.
"As Mr Parker said, a partial picture does the public a disservice, yet nothing in this speech did anything to assist a more informed picture or address public concern about the scale of the information being collected by the agencies.
"The fact he does not feel GCHQ's reach should be publicly discussed is in stark contrast to the US government's efforts to maintain public confidence by bringing further transparency and oversight to the reach of the NSA.
"People will rightly question why, if the US congress can publicly debate the reach of their agencies, the British public should be denied any details of what is happening here."
A Guardian News and Media spokeswoman 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."
Clare Algar, executive director of legal charity Reprieve, said: " It is hard to see how what we have learned over recent months or years shows that we have proper accountability for our security services - if anything, the situation is getting worse.
"The Justice and Security Act, passed this year, rolled out secret courts across the civil justice system which will help the Government cover up wrongdoing and avoid accountability.
"Ministers are already planning to use them in a case concerning UK involvement in the kidnap and 'rendition' of Gaddafi's opponents - along with their wives and young children - back to the dictator's prisons in 2004.
"The Intelligence and Security Committee - which it is worth remembering missed UK complicity in rendition altogether - continues to spend more time defending the security services in the media than actually holding them to account.
"The Government needs to stop eroding the freedom and independence of our courts, or this lack of accountability will not change."
Guardian editor Alan Rusbridger defended the newspaper's role in publishing the sensitive information, claiming it was stepping in where MPs had failed.
He told BBC Radio 4's World at One: "It's quite surprising to me that the number of MPs in this country who have said anything at all in the last four months can be counted on one hand - Malcolm Rifkind, Tom Watson, David Davis.
"So, if Parliament's not going to have this discussion and if the courts can only do this in private then I think absolutely it falls to the press to stimulate a discussion, which as I say through America, throughout Europe, is one that the public is intensely interested in."
Challenged on Mr Parker's suggestions that publishing the information was helping terrorists, he replied: "They will always say that. You read histories of intelligence and you go back to the 1990s and the security people were saying the same."
Mr Rusbridger said the newspaper was working "slowly and responsibly" through the information it had received and would publish more stories as it discovered them.