For all the fuss, Shayler is a minor player in the history of UK espionage

A voracious appetite for self-publicity and the Government's clumsy responses to him have ensured David Shayler will be forever linked in the public mind with the cloak-and-dagger world of espionage. But the precise extent of his importance and effect remains a matter of debate.

A voracious appetite for self-publicity and the Government's clumsy responses to him have ensured David Shayler will be forever linked in the public mind with the cloak-and-dagger world of espionage. But the precise extent of his importance and effect remains a matter of debate.

Spies such as Kim Philby, Guy Burgess, or Donald Maclean passed on information to the Soviet Union and inflicted considerable damage to Britain's security apparatus. Once discovered, they were whisked off behind the Iron Curtain by the KGB. Others who betrayed secrets to the Russians were, however, given immunity from prosecution and allowed to remain in Britain, until their secret was belatedly discovered. Sir Anthony Blunt is the most famous, or most infamous example of this. Melita Norwood, the ''granny spy" named by Soviet defector Vasili Mitrokhin is a more recent example.

Mr Shayler belongs to another type of renegade who could not be accused of dealing with the enemy. Their employers in MI5 and MI6 claim the enemy hugely benefits from what is being leaked and vital intelligence operations and assets are compromised. But Mr Shayler and others like him dispute this and maintain that they are acting on patriotic motives to expose abuse of power at the top.

The trend towards whistleblowing began with Peter Wright's book Spycatcher. The legal battles, which the Government lost, were a goldmine of publicity for the book and ensured Mr Wright could retire in Australia as a millionaire.

Mr Wright had failed to catch any spies, but he highlighted the intrigue among right-wing factions in the intelligence services to destablise and even bring down the Labour government of Harold Wilson. The allegations appeared to be backed up by people involved with the security services in Northern Ireland, such as Colin Wallace and Fred Holroyd. There was, however, considerable debate about their claims, and neither managed to accumulate the financial riches of Wright. Mr Wallace ended up in prison framed, he claimed, by the security services.

Cathy Massiter and Richard Tomlinson also exposed secrets from the intelligence services. Ms Massiter, who worked for MI5, revealed how the Campaign for Nuclear Disarmament had been infiltrated and spied on by her employers. She felt the full force of the threat of arrest and charging under the Official Secrets Act. She now lives quietly in the Home Counties with injunctions to keep her silent.

Mr Tomlinson served a brief period in prison before, like Mr Shayler, going into exile, after falling foul of MI6 over revelations he had threatened to make. Expelled from Switzerland, he lives in northern Italy. Mr Tomlinson is suspected of doing far more damage than either Ms Massiter or Mr Shayler. He is blamed for putting on the internet the names of more than 100 active agents some of whose lives, claim the security services, are now in danger.

Mr Shayler and his friendsbelieve the deal he is cutting with the authorities will mean there is little chance of a successful prosecution, and lucrative media deals arelikely to follow. But it is unlikely that he has any more secrets to tell.