"Al-Qaida is lapping it up," declared the chief of Britain's foreign intelligence agency, John Sawers, who is also known in official circles as "C" (like "M," James Bond's fictional superior in the same agency).
His colleague, Iain Lobban, said his staff was now seeing "near-daily discussion" by terrorist groups on how to avoid having their digital communications caught in the spying dragnet revealed by former U.S. National Security Agency contractor Edward Snowden.
Lobban, head of eavesdropping agency General Communications Headquarters, or GCHQ, told Parliament's intelligence and security committee that Snowden's disclosures had made his staff's job "far, far harder."
But pressed for specific examples of intelligence-gathering being compromised, Sawers and Lobban said they would provide such details only in a private session with lawmakers and not in the highly unusual televised hearing held Thursday.
It was the first glimpse most Britons have ever had of Lobban, a career spy, and of Andrew Parker, the director of MI5, Britain's domestic intelligence agency, who also appeared before the parliamentary committee.
Sawers is only slightly less mysterious, having caused a stir three years ago by delivering a rare public address defending the covert operations conducted by his agency, the Secret Intelligence Service, or MI6.
The three spooks' joint appearance followed an acknowledgment last month by lawmakers that a public debate on mass surveillance and civil liberties had become increasingly necessary in the digital age. Malcolm Rifkind, the chairman of the intelligence committee, called Thursday's hearing a significant step forward in greater transparency.
Dressed similarly in dark suits and looking like the middle-age white male civil servants they in fact are, the three spy chiefs dismissed criticism that Britain's intelligence services are inherently inimical to personal privacy and freedom.
"The suggestion that what we do is somehow compromising freedom and democracy - of course we believe the opposite to be the case," Parker said. "The work we do is addressing directly threats to this country, to our way of life and to the people who live here."
Parker, who has called Snowden's revelations a "gift" to terrorists, said a "team effort" by the three agencies had foiled 34 terrorist plots since the July 2005 suicide bombings on London's transport network, which killed 52 people.
Lobban forcefully defended the work of his agency, GCHQ, which earlier this year was cleared by the parliamentary panel of breaking any laws in collecting electronic data and using information provided by the U.S. through its vast spying networks.
"We do not spend our time listening to the telephone calls or reading the emails of the majority, of the vast majority. That would not be proportionate; it would not be legal; we do not do it," Lobban said.
But, he added, "certain methods should remain secret. . I don't think 'secret' means 'sinister.'"
All three bosses were plainly disapproving of Snowden's leaks and said that a similar breach of security in Britain was unlikely. Sawers criticized the newspapers that have published the leaks.
"I'm not sure that the journalists who are managing this very sensitive information are particularly well-placed to make those judgments," he said. "What I can tell you is that the leaks from Snowden have been very damaging. They put our operations at risk. It's clear that our adversaries are rubbing their hands with glee."
The Guardian, the British newspaper that broke the story, suggested that the U.S. and British intelligence services needed to put their own house in order.
"The disastrous loss of classified data was not the responsibility of journalists but of the intelligence community itself," the Guardian said in a statement. "We understand that the agencies will always warn that any form of disclosure has a damaging impact on their work. But this cannot mean the end of all questioning and debate."
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