The heads of the three UK intelligence agencies, MI5, MI6 and GCHQ, will be publicly grilled by MPs on the Intelligence and Security Committee later.

It is the first time they have appeared together to be questioned about their work, and it comes after the revelations of National Security Agency (NSA) whistleblower Edward Snowden.

Glenn Greenwald, a former Guardian journalist, feels the parliamentary system has, so far, failed to hold Britain's intelligence agencies to account.

He explained: "There was a huge suspicion-less system of mass spying that the British people and the American people had no idea had been built in their name and with their money.

"But I believe that that system can bring about real accountability if there's the political will. And there's lots of MPs who have praised the reporting that we've done and said that real reform is necessary.

"And they seem to be serious about using the proper powers of Parliament to make sure that these spy agencies are reined in and have some accountability," he added.

Sir David Omand, visiting professor at the war studies department of King's College London, and former director of GCHQ believes there "has been damage to public security".

He said: "They (the committee) are on show this afternoon every bit as much as the three heads of agency. They have to demonstrate they can satisfy the need for oversight, and satisfy parliament that they're doing a job that, in other areas of government, can be done with much more open means."

First broadcast on BBC Radio 4's Today programme on Thursday 7 November 2013.