Britain's enemies are "rubbing their hands with glee" over whistleblower Edward Snowden's National Security Agency (NSA) leaks, the MI6 chief said.
ITV News UK Editor Lucy Manning reports:
The leaks which revealed some of the surveillance activities of GCHQ and its American counterpart NSA were "damaging" and had "put operations at risk", Sir John Sawers told a group of MPs.
ITV News UK Editor Lucy Manning tweeted:
Sir John joined MI5 director general Andrew Parker and GCHQ director Sir Iain Lobban during the first public hearing of the parliamentary Intelligence and Security Committee (ISC).
Sir Iain said terrorists were moving to less vulnerable communication methods in the wake of the leaks.
He said activists in the Middle East and those"closer to home" had been monitored discussing ways of switching away from communications they "now perceived as vulnerable".
The head of listening post GCHQ also denied delving into "innocent e-mails and phonecalls".
ISC chair Sir Malcolm Rifkind quizzing Sir Iain, said many believe the "real cyber threat" comes from GCHQ seeking to collect data communications.
He added that the internet was an "enormous hayfield" and GCHQ was trying to access "those parts of the field that we can get access to and which might be lucrative in terms of containing the needles or the fragments of the needles we might be interested in, that might help our mission".
Sir Iain also suggested the leaks could help paedophiles avoid detection and the success of intelligence operations required the country's enemies to be "unaware or uncertain" of methods.
Mr Parker told MPs that MI5 had disrupted 34 terrorist plots "at all sizes and stages" since the London bombings on 7 July 2005.
He said "the vast majority" of the plots had "come from people who live here".