Ever since he was sacked as an MI6 agent in 1995, Mr Tomlinson is said to have harboured a grudge against elements of the British establishment.

His disillusion seems to have been fuelled by the decision of the then Foreign Secretary, Malcolm Rifkind, to deny the New Zealand-born spy an industrial tribunal, on the grounds it might jeopardise national security.

James Bond-type figure

Since then Mr Tomlinson has made a series of high-profile claims in the press about MI6 operations and fought against gagging orders to publish a book about MI5.

His current "pariah" image is a world away from that when he first signed up with the security services, of a text-book James Bond figure.

Cambridge University, where Tomlinson got a first

Mr Tomlinson was recruited into the clandestine world of foreign intelligence in 1991 after a stint working in the City of London.

With a Cambridge first in aeronautical engineering under his arm, and weekends spent training with a territorial division of the SAS, he must have looked an ideal candidate.

Working under the codename T, he was posted to Bosnia, Moscow and the Middle East.

But in 1995, at the end of his training with the Secret Intelligence Service, he was sacked. He was viewed as "unsuitable" - too much of a loose cannon to be trusted.

Speculation is that he ruffled feathers by raising concerns about Serbian donations to the Conservative Party, while attempts to infiltrate the a Middle East chemical weapons procurement network had gone wrong.

Suicidal depression

Mr Tomlinson's only right of recourse - to take his employers to industrial tribunal - was refused on the grounds of "national security".

He sank into a depression that was compounded by the death from cancer of his girlfriend. He is said to have told MI6 that he had become suicidal.

He went to live in Spain and, in an attempt to get back at his former employer, wrote articles for the British press.

This, alongside his threats to expose MI6 in detail on the Internet, saw Mr Tomlinson arrested on his return to the UK. In 1997 he became the first MI6 agent to be tried under the Official Secrets Act since George Blake in 1961.

He pleaded guilty and served six months in a British prison, an experience that intensified his depression.

Cried hypocrisy

He is reported as having reflected: "Always the deep anger would well up in me - fuelled by resentment at the circumstances that led to my imprisonment."

He was also struck by the "hypocrisy" of being tried in court when he had been refused a legal right of redress against his dismissal.

Once out of jail, Mr Tomlinson was back in the headlines, behind allegations designed to embarrass the British authorities. His claims included:

• Henri Paul, the driver who was killed alongside Diana, Princess of Wales, had been an MI6 agent. Mr Tomlinson formally presented his evidence to the judge investigating the crash in Paris.
• A plan to assassinate the Yugoslav president Slobodan Milosevic.
• The naming a British spy in the Bundesbank.
• Dominic Lawson, editor of the Sunday Telegraph, was an MI6 stooge.

David Shayler: Like Tomlinson, has called for reform of security services

Mr Tomlinson's case echoes that of David Shayler, the former MI5 agent now living in exile in France. Both men have urged reforms of the security services, including greater scrutiny by parliament.

But Mr Tomlinson, who now spends much of his time in Switzerland, has denied he is motivated by revenge. He said in a radio interview: "My dismissal is a long time in the past ... it has nothing to do with that."

For its part, the UK Government is keen to portray him as a deluded self publicist.

A Foreign Office spokesman said he was a "fantasist" without "moral scruples" living in a "fantasy world".