By Andrew Osborn
LONDON (Reuters) - Britain's intelligence chiefs will give their first ever public testimony on Thursday when they are cross-examined together in parliament about the case of former U.S. spy agency contractor Edward Snowden.
The evidence-gathering session comes amid calls for the government to step up oversight of its three main intelligence agencies after documents that Snowden leaked to the press exposed Britain's role in secret mass surveillance programs.
Those disclosures detailed Britain's close cooperation with the U.S. National Security Agency (NSA), embarrassing Prime Minister David Cameron and angering lawmakers in his ruling Conservative party who said they harmed national security.
The director of Britain's electronic eavesdropping agency GCHQ, the head of the domestic security service MI5, and the chief of the foreign Secret Intelligence Service, otherwise known as MI6, will all attend Thursday's hearing, which will be televised, albeit with a short delay for security reasons.
In the past, such hearings have been behind closed doors.
"(The intelligence chiefs) have traditionally operated behind a veil," a government spokesman said. "But they are more publicly available than they have been in many years. This is a step forward in terms of transparency."
People familiar with the agenda said the officials would be asked whether mass surveillance programs were a violation of privacy, and what impact the Snowden leaks had had on their work.
NO OPERATIONAL MATTERS
They will not, however, be asked to discuss operational matters - again, for security reasons - or to elaborate on the functioning of the surveillance programs.
Civil liberties groups, parts of the media and lawmakers from all parties have argued that Snowden's disclosures about the scale of GCHQ's monitoring activities show that it has become too powerful and needs to be reined in.
Cameron has rejected that idea, arguing that it is already subject to proper oversight and that its work needs to be kept secret to protect national security.
He has accused Snowden and newspapers that have published his information of assisting Britain's enemies by helping them to avoid surveillance by its intelligence agencies.
He has also threatened to stop future publications if necessary.
Thursday's session will last for about an hour and a half and extend to other topics including the agencies' current priorities and threats to Britain.
Organizers say the session was planned long before the Snowden affair erupted and was not a direct response to it.
Critics have dismissed the hearing as a public relations exercise designed to fend off calls for greater oversight and limit the fallout from the Snowden affair.
Malcolm Rifkind, chair of the parliamentary Intelligence and Security Committee that will conduct the hearing and a member of Cameron's Conservative party, says the session is proof that greater oversight is already happening.
(Editing by Kevin Liffey)