The heads of GCHQ, MI5 and MI6 are to appear before an government committee at 2pm today to give evidence relating to spy claims.
GCHQ director Sir Iain Lobban, head of MI5 Andrew Parker, and head of MI6 Sir John Sawers will all face questions from the Intelligence and Security Committee (ISC), which will be broadcast live on television.
The three intelligence chiefs are expected to face questions about the leaks by Edward Snowden, a former contract worker for the National Security Agency (NSA) in the US. In documents released to the Guardian newspaper earlier this year, GCHQ's Tempora programme was revealed to be capable of tapping into the communications of millions of citizens.
GCHQ's involvement with the NSA's PRISM spy programme was also brought to light through the leaks, and their relationship is expected to be the subject of questions today.
The heads of GCHQ, MI5 and MI6 are also expected to face questions regarding Edward Snowden and the extent of the surveillance, as well as how effective their programmes have been with regards to terrorist threats. Ongoing operations will not be questioned.
Former head of GCHQ, Sir David Omand, questioned the effectiveness of the committee on BBC Radio 4's The Today show this morning. "As a result of the revelations we know less about the people who are trying to harm us and we are therefore less safe," he said.
"I have argued for a long time that the government should have been more open about the purpose of intelligence and the general ways in an internet age you have to go about accessing intelligence. That debate is perfectly reasonable."
Glenn Greenwald, the Guardian journalist who helped bring these revelations to light, also criticised the ISC but believes that real accountability can still be achieved.
"I think the system has failed to exercise meaningful accountability up to this point because there was a huge suspicionless system of mass spying that the British and American people had no idea had been built in their name," Greenwald said. "But I think that system can bring about real accountability if there is the political will."
The ISC has said that the committee aims to "give an insight into the world of intelligence and the work the agencies do on behalf of the UK. (It is) a very significant step forward in terms of the openness and transparency of the agencies."