"It causes enormous damage to make public the reach and limits of GCHQ techniques," British Security Service Director General Andrew Parker said, using the initials of the Government Communications Headquarters, Britain's counterpart of the U.S. National Security Agency.
"Such information hands the advantage to the terrorists. It is the gift they need to evade us and strike at will," Parker, whose agency is better known as MI5, told an audience at London's Royal United Services Institute for Defense and Security Studies, a defense and security think tank.
"Unfashionable as it might seem, that is why we must keep secrets secret, and why not doing so causes such harm," Parker said in his first public speech since taking up his post as MI5 chief April 22.
He did not mention Snowden by name or cite British newspaper The Guardian, to which the former NSA contractor leaked a reported 58,000 or more GCHQ files in addition to NSA files.
Snowden claims he leaked the U.S. and British files, as well as files from other Western governments, to expose secret mass surveillance by the state.
Washington, London and the other exposed governments say the leaks were not in the public interest. A senior British official has said they amount to "a handbook for terrorists on avoiding detection," British newspaper The Independent reported.
Snowden, who perpetrated one of the biggest intelligence leaks in U.S. history, is a finalist for this year's Sakharov Prize for Freedom of Thought, to be announced by the European Parliament Thursday.
The prize is named after Soviet human-rights activist Andrei Sakharov.
Past recipients include anti-apartheid activist Nelson Mandela, who later became South Africa's president; Burmese opposition leader and current politician Aung San Suu Kyi; Russian human-rights group Memorial and Cuban democracy activist Oswaldo Paya, who died last year in a mysterious car accident.
Parker told the think tank Tuesday "several thousand Islamist extremists" who see "the British people as a legitimate target" are on British soil.
Among the terrorists are Britons themselves who traveled to Syria -- a hotbed of extremism and terror groups -- and then returned home, he said.
The number of such terrorists is in the low hundreds, intelligence sources told British newspaper The Daily Telegraph.
Parker said the terrorist threat now is no worse now than before, "but it has become more diversified ... more diffuse, more complicated, more unpredictable."
He also echoes messages given by U.S. National Intelligence Director James Clapper and NSA Director Gen. Keith Alexander, that domestic surveillance does not include unwarranted searches into innocent people's private lives, and said government checks and balances keep surveillance within the law.
"The law requires that we only collect and access information that we really need to perform our functions," Parker said.
"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 in remarks quoted by The Independent.
Even when it comes to suspects, "Knowing of an individual does not equate to knowing everything about them," Parker said. "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 time."
He added: "The idea we either can or would want to operate intensive scrutiny of thousands is fanciful. This is not East Germany or North Korea, and thank goodness for that."