REVELATIONS by the former US analyst Edward Snowden have put UK counter-terrorism operations at risk, spy chiefs have warned.

In their first public evidence session they said terror suspects were planning to switch their communication methods as a result of the leaks.

Sir John Sawers, head of MI6, said that the released information had "put our operations at risk".

He added: "It's obvious that our adversaries are rubbing their hands in glee. Al Qaeda are lapping it up."

The bosses of the UK's three spying agencies, MI5, MI6, and the eavesdropping agency GCHQ, also revealed that 34 terror plots, some aimed at mass casualties, have been disrupted in the UK since the London bombings on July 7, 2005.

They warned several thousand people in the UK were involved in, or supported, violent extremism.

The historic session took place in front of Westminster's Intelligence and Security Committee, whose members are handpicked by the Coalition Government. In previous decades even the names of those in charge of MI5 and MI6 would have been top secret. Yesterday's session, however, follows growing questions over the security agencies' role and powers.

Such was the sensitivity around their evidence that it was broadcast through the parliamentary television system following a two-minute delay in case anything emerged that could have affected national security.

The measure proved unnecessary, however, in part because the spy chiefs refused to answer a number of questions from MPs and peers.

Each denied complicity in torture through foreign allies, and they told MPs that if the facts on the ground were uncertain they would "wake the Home Secretary up and ask them one way or another".

Sir Iain Lobban, head of GCHQ, which has been at the centre of the controversy sparked by the documents released by Mr Snowden, insisted his agency was not involved in "snooping" on the public.

"If you are a terrorist, a serious criminal, a proliferator, a foreign intelligence target or if your activities pose a genuine threat to the national or economic security of the UK, there is a possibility that your communication will be monitored," he said.

But ordinary people, who were not in contact with anyone of the above list, were not being monitored, he declared.

"We do not spend our time listening to the telephone calls or reading the emails of the majority, the vast majority that would not be proportionate, it would not be legal. We do not do it," he said.

Sir John also defended MI6 activities, insisting there was no way his agents would act outside the law. There was, he said, "very strong ethical standards" across all three intelligence agencies.

In his evidence, Andrew Parker, the new head of MI5, predicted an end to terrorism in Northern Ireland. He said he expected that those involved in all forms of terrorism in Norther Ireland, including dissident Republicans, would either end up jailed or "give up".

His comments follow a spate of letter bombs sent by dissident Republicans to targets including Northern Ireland Secretary Theresa May in recent weeks.

He also warned that so-called "terrorist tourism" - in which people travel abroad for training before returning to the UK - is "a very important strand" of the threat facing the UK.

The Syrian civil war has seen hundreds of British nationals looking for the opportunity for jihadi activity, he added.

Many of these have come into contact with al Qaeda-supporting groups before returning to the UK, he said.