Britain's top spy chiefs say the intelligence leaks by Edward Snowden have left its enemies "rubbing their hands with glee" and caused terror groups to change the way they communicate.
In a rare televised appearance, the heads of foreign spy agency MI6, its domestic counterpart MI5 and electronic eavesdropping agency GCHQ also denied that Britons were subject to mass surveillance.
In their evidence to parliament's Intelligence and Security Committee, held under tight security and with a two-minute time delay to prevent accidental disclosures, MI6 boss John Sawers said Snowden's revelations of mass US and British surveillance programs were a gift to Al-Qaeda and other terrorists.
"The leaks from Snowden have been very damaging; they put our operations at risk. Our adversaries are rubbing their hands with glee. Al-Qaeda is lapping it up," Sawers told the committee.
GCHQ boss Iain Lobban added that the revelations by the fugitive US leaker had led "terrorist groups" in the Middle East, Afghanistan and elsewhere to modify the way they communicate.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"We have intelligence on (and) we have actually seen chat around specific terrorist groups, including closer to home, discussing how to avoid what they now perceive to be vulnerable communications methods," Lobban said.
Snowden, a former US National Security Agency (NSA) contractor, who has been granted temporary asylum in Russia, has revealed massive US electronic surveillance programs in recent months, sending shockwaves around the world.
The leaks have strained Washington's ties with its allies over suggestions that it has eavesdropped on dozens of world leaders, including by tapping the mobile phone of German Chancellor Angela Merkel.
Thursday's hearing marked an unprecedented joint public appearance for the heads of the three British intelligence agencies, who could be seen on television sitting in a row as they answered questions from MPs.
They insisted that they worked hard to balance national security with citizens' right to privacy.
"We do not spend our time listening to the telephone calls or reading the emails of the majority. It would not be proportionate. It would not be legal. We do not do it," Lobban told the committee.
MI5 director general Andrew Parker insisted that the work of the intelligence services was a "proportionate" response to the terrorist threats faced by Britain.
"The suggestion that what we do is somehow compromising freedom and democracy -- of course we believe the opposite is the case," Parker told the committee.
"The work we do is proportionate, judged against the necessity of protecting against these threats."
Sawers emphatically denied that British agents used torture as a means of countering threats to national security, stressing they "only operate within the framework of the law."
