Shayler: hero of free speech, or the spy who loved himself ?

"What would MI5 want with someone like me, a journalist who printed extracts from Spycatcher when I was at college, someone who had been active on the fringe of left-wing politics?" David Shayler was an unlikely "spook" and his own account of his reaction to his acceptance into the Security Service shows that no one was more surprised at the appointment than the applicant himself, a Middlesbrough football fanatic and a one-time Dundee University leftie.

Today, the former MI5 operative is facing a sentence of up to six years in jail after a jury at the Old Bailey convicted him of breaching the Official Secrets Act by selling confidential documents to a Sunday newspaper.

Shayler's relationship with the intelligence services began, after a failed attempt to break into journalism, when his mother noticed a strange recruitment advertisement in a newspaper. Headlined "Godot isn't coming" – a reference to Samuel Beckett's absurdist play Waiting for Godot – it appealed to frustrated people who were "stuck in a rut and unable to progress".

The language in the job ad was hardly the material of Ian Fleming. "Use your strong interpersonal skills to move to a non-commercial organisation where an interest in current affairs is important," it said.

Shayler, now 36, sent his CV to an anonymous PO box number and was called to an unmarked building in Tottenham Court Road, central London, for an interview with a man in a pin-stripe suit.

He signed the Official Secrets Act during the first interview, before being vetted and hired in 1991.

The security services may now regret not taking a reference from Shayler's headmaster at John Hampden grammar school, High Wycombe, Buckinghamshire, who once described Shayler as "a born rebel who sails close to the wind".

Or speaking to The Sunday Times newspaper, where he worked briefly as a trainee journalist before being released for not being "up to scratch".

The renegade agent has attempted to style himself as a whistleblower who sacrificed his career for the principles of democracy and free speech, but his appetite for self- publicity has not always won him friends.

He became caricatured as "The Spy Who Could Not Keep His Mouth Shut" and "The Spy Who Loved Himself".

At this Old Bailey trial, Shayler, conducting his own defence, attempted to persuade the jurors that he had acted as a "patriot" in selling confidential files for £40,000 to The Mail on Sunday in 1997.

He told the court: "It should be a fundamental principle of a fair society that people are allowed to expose the wrongdoing of the security services, particularly when they are a threat to our liberty. I was seeking to expose the truth.

"I'm not the first person in history to stand up and tell the truth and be persecuted, and I doubt I'll be the last."

The jury was given access to the leaked documents, some marked "Top Secret".

The files included a 135-page memorandum on links between the Provisional IRA and Libya between 1971 and 1996, and an MI5 investigation of right and left-wing "subversive" groups in the UK.

Other papers included a report from an unnamed foreign intelligence agency on the Lockerbie bombing, dated 1994, and a memo from 1992, headed "Moscow gold", on Soviet funding of the Communist Party of Great Britain.

When he made his revelations, Shayler sensationally claimed that the former Labour minister Peter Mandelson had been under surveillance by MI5 as a suspected Soviet agent.

During his trial, Shayler told the jury that his disclosures had not placed any of his former colleagues in danger.

But Nigel Sweeney QC, for the prosecution, said that even one piece of classified information could be the "final piece in the jigsaw" allowing hostile countries or organisations to identify British agents.

At an earlier hearing at the House of Lords, the prosecution had said the disclosures could have resulted in the potential "deaths of 50 agents".

During his five years at the security services, Shayler worked for the vetting section, the anti-subversion section, a counter Irish Republican section, which tracked suspected terrorists on the British mainland, and finally the Middle East section, where he claims he learnt of an MI6 plot to blow up the Libyan leader Muammar Gaddafi.

He resigned in September 1996, his friends claim, because he found the security services too regimented and bureaucratic, and was appalled by the waste of public money and the illegal acts committed by bungling agents.

Others suggest he was annoyed that his superiors had turned him down for promotion.

Shayler's disclosures were in the press within a year of his resignation.

Before the story had hit the news-stands, the former agent and his girlfriend Annie Machon, a former MI5 colleague and Cambridge University graduate, fled the country for Amsterdam and then for France. The couple later settled down to living in what Shayler described as "political exile" at a remote farmhouse in La Creuse.

During late 1997 and 1998, more Shayler revelations claimed that MI5 could have prevented the IRA bombing in Bishopsgate in London in 1993 and the bombing of the Israeli embassy in London in 1994.

In August 1998, he was arrested by French police and spent four months in jail while the British Government tried unsuccessfully to have him extradited to face charges under the Official Secrets Act.

He finally returned to Britain in August 2000 and pledged to mount a legal defence based on his having acted in the "public interest".

Throughout the entire episode, Ms Machon has remained steadfastly loyal.

After yesterday's verdict, she said: "David is a whistle-blower pure and simple. I'm shocked at the verdict. He deserves to be protected, not prosecuted."

She noted that the jury had spent more than three hours considering its verdict.

Shayler's stance has also won him the support of the civil rights group Liberty, whose director, John Wadham, has acted as his solicitor. Mr Wadham said the case would be taken to the European Court of Human Rights in Strasbourg.

But while the former agent awaits sentencing today, Home Office ministers will be hoping that the Shayler episode does not hinder the security services' recent attempts to open their doors to a wider cross-section of British society.

The end of the trial coincided with the publication of a new MI5 booklet, expressing a renewed commitment to diverse recruitment.

The booklet says: "It means attracting a diverse range of staff from all backgrounds and from all over the UK, so that we can benefit from their different perspectives, experiences and approaches."

From a newspaper deal to a court battle: how the story unfolded

August 1997: Shayler supplies details of MI5 operations to The Mail on Sunday, accusing the Government of keeping files on senior Labour politicians, including Peter Mandelson and Jack Straw, for which he is paid £40,000. He leaves Britain straight after publication.

July 1998

Shayler accuses MI5 of failing to react to information about a terrorist attack on the Israeli embassy in 1994. He claims MI6 officers planned an assassination of the Libyan leader Colonel Gaddafi in 1996.

August 1998

Shayler is arrested in France and held in Paris's La Sante jail without charge for four months. In November, a French court rejects British attempts to extradite him and efforts to charge him are dropped. Shayler threatens to make fresh revelations about the secret services.

December 1999

Shayler launches campaign to return to the UK

February 2000

Government issues writ for breaches of confidence, contract and copyright laws on files held by MI5 and MI6.

July 2000

The High Court quashes an order made in March requiring two newspaper editors to hand over documents and e-mails sent them by Shayler. The judge said orders risked stifling investigative journalism unless there was "compelling evidence" they were in the public interest.

August 2000

Shayler arrives back after the Crown Prosecution Service says it will not oppose bail. He is charged under the Official Secrets Act within hours.

May 2001

The High Court refuses Shayler permission to use a public-interest defence against accusations that he revealed state secrets. In November, he is given leave to appeal to the House of Lords.

March 2002

Law lords uphold the High Court's ruling and unanimously decide the Official Secrets Act does not conflict with European Convention on Human Rights' guarantee of freedom of expression.

October 2002

Shayler goes on trial at the Old Bailey on three counts of exposing official secrets. He is found guilty yesterday of disclosing information, documents and information from telephone taps.