It was an extraordinary trial. All the prosecution had to do was prove that David Shayler had copied secret documents from MI5 files and handed them to a national newspaper - a charge he did not deny, writes Richard Norton-Taylor.
When Shayler told the court he was on trial for damaging national security, the trial judge, Mr Justice Moses, interrupted immediately.
"Don't treat the jury as fools", he told Shayler. "You are not being tried for damaging national security".
Shayler's argument that it was a crime to expose a crime was ruled irrelevant.
Shayler did away with his lawyers and defended himself because, in law, he had no defence. In pre-trial hearings, the high court, court of appeal, and the law lords all ruled that the Official Secrets Act imposed a lifelong duty of confidentiality preventing members and former members of the security and intelligence agencies from saying anything about their work without official authority. The courts dismissed Shayler's argument that the secrets act was incompatible with the Human Rights Act, which enshrines the principle of freedom of expression.
In pre-trial hearings that can only now be disclosed, the judge ruled that Shayler had to show him and the prosecution in advance what he intended to tell the jury. David Blunkett, the home secretary, and Jack Straw, the foreign secretary, signed public interest immunity (PII) certificate, designed to prevent any information about security and intelligence from being disclosed in court.
Shayler was charged with passing documents and information to the Mail on Sunday, which on August 24 1997, revealed that MI5 held files on Peter Mandelson, Jack Straw, John Lennon, and other people it once considered to be subversive.
Shayler also criticised MI5's handling of an investigation into Khalifa Ahmed Bazelya, a Libyan intelligence officer in London, and what he called the agency's "excessive bureaucracy" and "drinking culture".
The day before publication, Shayler fled abroad with his girlfriend Annie Machon, also a former MI5 officer. A year later, he was arrested in Paris by French police on a British extradition warrant.
He had been in prison for nearly four months when a French judge ruled that the extradition demand was politically motivated and ordered his release. Last year Shayler returned to Britain saying he intended "to challenge the cover-ups and complacency that have followed my disclosures".
He was arrested at Dover, charged, and bailed. He continued to make allegations.
Shayler sent a file on the plot to Jack Straw, to parliament's security and intelligence committee, and to the police who, after a lengthy investigation, dropped the issue.
He said MI5 could have stopped IRA bombers six months before they bombed Bishopsgate in the City of London in 1993.
In a further allegation, Shayler said MI5 knew before the event about a plan to bomb the Israeli embassy in London in 1994. Jack Straw, the home secretary, said he understood why Shayler could have come to that conclusion but added that MI5 could not have prevented the attack.
None of these allegations, on the face of it more serious than those that appeared much earlier in the Mail on Sunday, were the subject of the charges against Shayler. The prosecution, MI5, and MI6, made every effort to prevent him from mentioning them in front of the jury. The prosecution only had to show that Shayler had passed information - any information - about security and intelligence to journalists without official authority, not that he had damaged national security.
It did not have to prove whether Shayler's allegations were true or not. Shayler's motives were irrelevant.
The prosecution also dwelt on the fact that the Mail on Sunday had paid Shayler nearly £40,000.
One question that was asked on the sidelines of the trial was why MI5 recruited David Shayler in the first place. After he failed a Sunday Times journalist training course, he responded in 1991 to a recruitment advertisement placed by MI5 that asked: "Are you waiting for Godot?".
· A fuller version of this article will appear in Tuesday's Guardian