The way it works is quite simple. If somebody mysteriously disappears from circulation and you think they have been arrested or detained by somebody, then you may at any time ask a judge to issue the writ. In urgent cases the application is even made by phone.

The writ itself is a court order requiring the person who has another in his custody to produce the prisoner in court and explain the legal basis for the detention.

The writ is rarely used these days, but its importance remains fundamental. The right not to be imprisoned 'except by the lawful judgment of his peers or by the law of the land', as the Magna Carta puts it, is a basic one. It has no meaning unless there is a procedure to safeguard it.

Habeas corpus is the reason why in common law countries there is no Gulag Archipelago, no 'desparecidos', no 'lettres de cachet', and no places where people against whom no offences have been proven can be detained indefinitely at the whim of officials - except perhaps Guantanamo Bay. As the Centre for the Preservation of Habeas Corpus puts it: 'Habeas Corpus empowers the individual in holding accountable the state's awesome power to restrain liberty.'

This is the procedure at the background of the case involving four individuals accused of trying to 'use the court and the media' to press the Independent Commission Against Corruption to release a woman who was, it turned out, in the commission's witness protection programme.

Now I know no more about this case than you can gather from the newspapers and although it is now legal to do so, I do not wish to express or imply any opinion about the innocence or guilt of those accused. I am, though, concerned about what this case tells us of attitudes to the writ of habeas corpus and its role in restraining the awesome power of the ICAC.

Readers will have gathered that the writ was invented long before the appearance of that contemporary institution, the witness protection programme. No doubt the coexistence of the two presents some practical difficulties. I am not sure, though, that they were happily solved in the hearing which led to this case.

The answer to the writ was provided, apparently, by an ICAC officer who turned up and asserted that the woman whose presence was demanded was in the ICAC's witness protection scheme, and consequently the writ should be discharged and the proceedings kept secret. The judge agreed.

Is this good enough? To start with, it is not an abuse of the writ to point it at a branch of the government. On the contrary, that has always been its leading role. The Act of 1679 is specifically said to be for the 'relief of all persons imprisoned for ... criminal or supposed criminal matters'.

The ICAC may feel participation in its witness protection programme is not a form of imprisonment, but that is a matter for the court to decide. It would be nice to believe all participants in the programme are motivated by a zeal for the public good and law enforcement, but this is naive.

Like all law-enforcement institutions, the ICAC has been known to use that prosecutorial technique in which the victim is offered a choice of roles in the forthcoming proceedings: witness for the prosecution or defendant.

It may be that some participants, offered this choice in the less oppressive air of a court room rather than an interrogation cell, will wish to reconsider their choices. No doubt that would be attended by some inconvenience to the ICAC. But that is surely better than the alternative we seem to be contemplating at the moment, which is that the writ can effectively be thwarted by the unsupported word of the organ of the state at which it is directed.

Of course, the ICAC is a well-intentioned body which considers its work important. But as an American judge once put it: 'Experience should teach us to be most on our guard to protect liberty when the government's purposes are beneficent.'

Kevin Sinclair is ill but he will continue his On the Spot forum