The soothing surroundings of the vast atrium of Portcullis House are the meeting place of choice for today's MPs. It is a world away from the darkened book-lined corridors and rabbit-warren staircases of the House of Commons across the road, the traditional home of Westminster intrigue.
But it was in Portcullis House that plain brown envelopes, containing explosive documents about illegal immigrants working as security guards, Labour MPs preparing to rebel against the Terror Bill, and the Home Secretary's fears that the economic downturn would lead to a crime wave were discreetly passed to journalists.
The resulting stories, over a period of a year, were acutely embarrassing for Jacqui Smith and her department, but were also used to piece together the identity of the civil servant accused of leaking information to Damian Green.
The practice of using a brown envelope to pass on information is commonplace in Westminster. At any one time, there are hundreds of MPs, researchers, journalists and visitors at Portcullis House, and the handing over of an ordinary envelope would rarely be noticed. As the IoS reveals today, the practice stems from a real concern that their movements are being monitored by MI5 or Special Branch.
Last Thursday's raid by nine anti-terrorist police officers on Mr Green's office, just off the Portcullis House atrium, has triggered accusations of contempt of parliamentary privilege.
In four co-ordinated raids at his home and offices, anti-terror police seized the MP's computers, mobile phone, BlackBerry and bank statements – as well as rifling through old love letters between Mr Green and his wife.
But the revelation that the offices of senior frontbenchers are routinely swept for bugs will send shockwaves through Westminster. It has serious repercussions for the operation of the Wilson doctrine, the convention that protects MPs from phone-tapping.
In 1966, following a series of allegations of bugging of MPs' telephones, the Prime Minister, Harold Wilson, ordered a ban on phone-tapping on all MPs. Yet the doctrine has failed to keep pace with modern technology, and MPs fear there is a "wide-open door" to security services listening to the conversations and reading the content of their emails, perfectly legally.
The doctrine covers only the intercept of communications – tapping phone lines or snatching data from mobile phone conversations, as well as the intercept of unopened emails and post. What is not covered are already opened emails and post, and, crucially, listening devices planted in an MP's office. One intelligence expert said it was possible for legally available software to be planted on a computer that copies all emails sent from that address.
It emerged earlier this year that Sadiq Khan, the Labour MP for Tooting, was bugged while visiting his constituent Babar Ahmad, a terror suspect, at Woodhill Prison, Milton Keynes. An official inquiry found that correct procedures were followed. National security justified the surveillance, it was claimed.
In February, the Home Secretary refused to tell the Commons how many MPs have been bugged during Labour's time in government. She announced a review of codes of practice so that discussions between MPs and constituents would remain confidential, although this would not cover the current case.
A Westminster source said last night: "MPs need to take precautions. The Damian Green case shows they are vulnerable to arrest, even if the information is not a threat to national security. Sweeping of offices for bugs may be one precaution, but if something is of great sensitivity, it is safer to pass things on in person."
Security experts said it was possible that the Speaker, Michael Martin, and Serjeant at Arms, Jill Pay, did not know if bugging devices were planted in MPs' offices in the Palace of Westminster.
One security expert familiar with leak investigations said that officials – led by the Home Office's permanent secretary, Sir David Normington – and police may have used a more prosaic way of identifying the civil servant suspected of leaking stories: by simply feeding him or her misleading information. After a leak was first suspected, in November last year, officials would have had as many as 15 suspects with access to the sensitive information.
Investigators may have fed a wrong story to one suspect to see if it appeared in the press, the expert said. It was only then that they would have identified the alleged mole and the police would have gone to arrest them.
A 26-year-old assistant private secretary was arrested on 17 November. Mr Green's arrest, on suspicion of aiding, abetting, counselling or procuring misconduct in public office, came 10 days later. The seizure of computers and mobile phones suggests that police are looking for evidence that the MP asked for specific leaks.
The former shadow home secretary David Davis, a friend of Mr Green, said last night it was outrageous that the matter had not been dealt with through internal disciplinary procedures at the Home Office. "We will be pressing the question as to exactly what ministers knew and at what time," he said.
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