Andrew Parker and John Sawers, heads of Britain's domestic security agency MI5 and foreign spy service MI6 respectively, appeared along with Iain Lobban, director of electronic surveillance agency GCHQ, to take questions from Parliament's Intelligence and Security Committee on Thursday.

The trio were questioned on the work of their agencies, their current priorities and threats to the UK. They did not, however, comment on ongoing operations.

Amid continuing revelations about the scope of US and British spying, fed by the leaks from former National Security Agency (NSA) contractor Edward Snowden, Parker defended the UK intelligence community.

"The suggestion that somehow what we do is somehow compromising freedom and democracy - we believe the opposite to be the case," he said.

Parker has previously described the leaks as a "gift" to terrorists.

MI6 chief Sawer echoed this sentiment. "The leaks from Snowden have been very damaging, they've put our operations at risk. It's clear that our adversaries are rubbing their hands with glee, al Qaeda is lapping it up,” he told lawmakers.

GCHQ boss Lobban said that the revelations from Snowden led to a change in behavior on the part of the terrorist groups the agency monitors.

Lobban said his spies had picked up on "near-daily discussion" of the revelations among GCHQ's targets. He said they were "discussing the revelations in specific terms of the communications packages which they use, and communications packages they wish to move to."

Lobban denied that the agency was listening to the majority of domestic email and telephone conversations, saying: "It would not be proportionate, it would not be legal, we do not do it."

The revelations have sparked a global debate over privacy and surveillance. It has also strained Washington's ties with its allies over suggestions that it has eavesdropped on several world leaders, including German Chancellor Angela Merkel.

The committee previously has taken testimony in private from the security chiefs. Thursday's hearing was televised with a slight delay.

hc/rc (Reuters, AFP, AP)