At a time when MI5 is having to counter the threat posed to this country from an estimated 2,000 home-grown terrorist suspects, the Max Mosley affair is both deeply embarrassing and an unwelcome diversion.

Although it would take only the most devoted conspiracy enthusiast to imagine that the Security Service, with only 3,500 staff members at its disposal, would want to dedicate intelligence officers to set up the head of Formula 1 motor racing, the involvement of the wife of an MI5 officer in the sado-masochistic orgy that brought about his downfall has raised serious questions about the organisation's vetting system.

MI5 has its own vetting department, it does not rely on outsiders checking on its staff, and the questions posed to all members of the Security Service, initially when they join and then at frequent intervals throughout their careers, are supposed to be among the toughest and most comprehensive in Whitehall. Recruits and experienced staff alike are put through the so-called developed vetting process which includes detailed questions and checks on an individual's relationships, family life and background and even any "inappropriate" contacts with the media.

The MI5 officer indirectly involved in the latest revelation about Mr Mosley's sex session with five prostitutes has resigned. But there are questions that have to be asked: did he know his wife was a prostitute, was he aware that she had played a role in the downfall of Mr Mosley, did he have a hint that she had gone to the News of the World with the story when she realised he had booked a sex session with five prostitutes?

If he had had any knowledge of his wife's secret life as a prostitute and had failed to tell his employers, he would have been in serious breach of MI5's code of conduct, which relies on its staff's integrity and honesty. No one seems to know the answers to these questions at this stage.

MI5's vetting procedures have not always been perfect. The case of David Shayler, the MI5 officer who joined the Security Service after an unsucessful bid to be a journalist on The Sunday Times, is another example where the organisation was severely embarrassed. Shayler was convicted of breaching the Official Secrets Act when he went to the Mail on Sunday with tales of alleged MI5 plottings after he had left the service. His potential for rebellion had not been spotted by the vetters during his five-year career in MI5.

In the lastest scandal, MI5 will want to maintain a discreet silence, not just because it's against its policy to talk about individual staff members, past or present, but because it feels its name has been unfairly cast into newspaper headlines in an affair that has already become a tabloid cause celebre. It has not been a good day for Britain's chief terrorist watchdog.

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