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 US spy agency contractor Edward Snowden.
"(The intelligence chiefs) have traditionally operated behind a veil...This is a step forward in terms of transparency.
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 programmes.
Those disclosures detailed Britain's close cooperation with the US National Security Agency (NSA), embarrassing Prime Minister David Cameron and angering lavesdropping agency GCHQ, the head of the domestic security service MI5, and the chief of the foreign Secret Intelligenawmakers in his ruling Conservative party who said they harmed national security.
The director of Britain's electronic ece 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 programmes were a violation of privacy, and what impact the Snowden leaks had had on their work.
They will not, however, be asked to discuss operational matters - again, for security reasons - or to elaborate on the functioning of the surveillance programmes.
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.