Britain's spymasters step out of the shadows The heads of the UK security services have long been the most secretive officials in the British state, people who almost never make a public appearance. But at 2pm on Thursday, history will be made when they step out of the shadows and appear live on TV before parliament's Intelligence and Security Committee. It was only in 1992 that the name of the head of MI5 was made public. It was only two years later that the UK government officially acknowledged that MI6 existed. Since then, the service chiefs have regularly give evidence to parliament - but strictly in private. Today, however, the three heads - Sir John Sawers, the chief of MI6; Sir Iain Lobban, the head of GCHQ; and Andrew Parker of MI5 - will appear before the ISC in a 90-minute open session. "I am not sure the heads of the services are going to find this an easy experience," says one MP on the ISC, which is made up of MPs, peers and former civil servants. These people didn't take on their jobs to do live speeches and hearings. Indeed, until very recently, we didn't admit these people existed. The decision to hold the open hearing is part of the beefing-up of the ISC's remit and independence, says Sir Malcolm Rifkind, the committee's chairman and a former foreign secretary. Sir Malcolm says the ISC now has significant new powers, in particular the right to send its staff into the intelligence services" headquarters and examine any material they wish. "The idea that the agencies are allowing outsiders into their premises like this is remarkable," he says. However, the timing of Thursday's hearing is also important. It comes as the secret world reels from allegations over the work of GCHQ and the role it plays alongside the US National Security Agency in hoovering up huge quantities of personal data on the internet. As a result, there is certain to be a strong focus by the ISC on Sir Iain Lobban, a reclusive figure whose Cheltenham-based agency is by far the most reticent of the three in its dealings with MPs and journalists. The ISC may want to know how much damage Sir Iain believes the revelations by Edward Snowden, the former NSA contractor, have done to UK intelligence. They may well ask whether Sir Iain agrees with the assessment by his predecessor, Sir David Omand, that the Snowden revelations are "the most catastrophic loss to British intelligence ever, much worse than Burgess and MacLean in the 1950s." But the ISC will also want to know whether Sir Iain accepts legislation is now needed to give better ministerial and parliamentary oversight of GCHQ activities. One ISC member says there has been "vigorous debate within the committee" on these issues as it prepares to make recommendations. For the other two heads of service, there will be less pressure. Mr Parker's agency, MI5, is widely seen as having had considerable success in preventing jihadist bomb plots across the UK in recent years. MI6 was for a long time living under the shadow of its flawed 2002 assessment that Iraq possessed weapons of mass destruction. On Thursday, questioning of Sir John Sawers is likely to be on current issues, in particular how he sees the evolving jihadist threats arising out of Syria and the Maghreb. Some commentators believe the hearing will test the ISC as well as the intelligence chiefs. This is because some believe it is unable to hold the services to account, despite its new powers. Alan Rusbridger, editor of the Guardian, which has published much of the Snowden leaks, said in a recent article that the committee chairman was not "to put it mildly, a child of the digital age." He says Sir Malcolm, like his counterparts in the US Congress, "would have struggled to understand" some of the documents on GCHQ activity leaked by Mr Snowden. But Sir Malcolm is confident that the ISC is becoming a robust interrogator of the security services. "Thursday's hearing is not going to be some kind of scripted event," he says. There will be time for follow-up questions. The agency heads are not going to know those questions in advance. Sir John Sawers, chief of MI6, the foreign intelligence service At 57, Sir John will be the most comfortable of the three service chiefs when they appear before the ISC on Thursday. After a lengthy diplomatic career, which took him to some of the highest posts in the Foreign Office, he is well used to engaging with politicians and appearing at high profile events. Sir John's four-year tenure at MI6 has been troubled, partly because he ended up having to manage legal challenges against the agency arising out of its previous work in Iraq and Libya. He is respected in Downing Street and is seen as the leading UK government figure on Iran policy. Andrew Parker, director-general of MI5, the domestic security service After just six months in the job, Mr Parker triggered controversy last month with a toughly worded speech that effectively attacked The Guardian for publishing documents relating to GCHQ's operations. In that speech, he said the leaks by NSA contractor Edward Snowden had caused "enormous damage" to UK national security. Mr Parker, 51, has a quietly spoken manner which colleagues say exudes a tough inner streak. He was director of counter terrorism at MI5 on the day al-Qaeda murdered 52 people in London on 7 July, 2005. MI5 has won considerable credit in Whitehall for the way it has contained jihadist threats since 2005. Sir Iain Lobban, director of GCHQ, the cyber-intelligence service Sir Iain has headed GCHQ since 2008. He is the most reclusive of the three heads of service and the only one who is completely unknown to the British media. Yet he is also the agency chief with most questions to answer about the way his organisation functions. GCHQ is at the centre of a huge political controversy because of the Snowden leaks. It is collaborating with the US National Security Agency in hoovering up huge quantities of personal data on the internet.