The most insidious form of political control on the media, however, is not external constraint but self-censorship. I am not referring here to any readiness by the media to check their own sometimes discreditable behaviour, intrusions into privacy, misstatements of fact or unwillingness to present diverse views and to foster democratic debate. That (in the case of newspapers) is supposedly the job of the Press Complaints Commission, which replaced the Press Council in 1990 as a voluntary self-regulating body. The likes of Fergie and David Mellor might be forgiven for regarding it as a watchdog without teeth.

The most institutionalised method of self-censorship is the D Notice system (short for Defence Notices). They are a unique peacetime arrangement of voluntary suppression of certain categories of information on the advice — not orders — of the Government. The system was established in 1912 and continues to this day. The justification for the system, as stated in the official guidelines, is as follows: Hostile intelligence services draw on information from a variety of sources both overt and covert, and by piecing it together can build up a composite picture of a subject. The dissemination of sensitive information can make their task easier and put national security at risk. It can also be of value to terrorist groups who lack the resources to obtain it through their own efforts. For these reasons there are dangers inherent even in the publication of information covered by D Notices which has already appeared elsewhere. It is strongly requested that there should be no elaboration, nor confirmation or denial, of the accuracy of items published elsewhere, without reference to the [D Notice] Secretary.

There are currently eight general [kinds of] D Notices (which, incidentally, used to be secret information themselves, but were made public in 1982):

Defence plans, operational capability, state of readiness and training Defence equipment Nuclear weapons and equipment Radio and radar transmissions Cyphers and communications British security and intelligence services War precautions and civil defence Photography etc. of defence establishments and installations

There is no direct relationship between the D Notice system and the Official Secrets Act; the latter has legal force, the former does not. As the official guidelines say, the D Notice system is entirely voluntary and has no legal authority; the final responsibility for the decision whether or not to publish lies solely with the editor or publisher concerned. However, the guidelines also state pointedly that the D Notice system is a useful reminder of the legal sanctions which may be brought to bear if an editor or producer oversteps the mark. Moreover, pressure to comply can be overwhelming. When in the early 1980s Granada TV made a documentary about the Official Secrets Act, the D Notice Committee asked them to exclude the address of the Government's Communications Headquarters (GCHQ) at Cheltenham. Granada objected, because the address was in Whitakers Almanac and other registers and was therefore public knowledge. However, the IBA — the body which then allocated broadcasting franchises and monitor ed commercial programmes — intervened and ordered Granada to cut the reference out of the programme. (The Government's subsequent ban on trade union membership at GCHQ and the resulting publicity, strikes and court cases, ironically, ensured that the existence and location of GCHQ rapidly became household knowledge.)

The Notices are issued and amended on the authority of the Defence Press and Broadcasting committee (DPBC), which is made up of officials from relevant government departments (e.g. Defence, Foreign and Home Offices) together with representatives of the press and broadcasting organisations. It is chaired by a senior MoD civil servant. The D Notices are sent out to national and provincial newspaper editors, radio and TV companies, and to some publishers of books and periodicals. When editors know that certain information falls under a particular D Notice they simply exclude it; when they are uncertain, they may seek advice. The Secretary of the D Notice Committee, Rear Admiral Higgins, receives on average one phone call a week from editors seeking guidance on potentially sensitive material; he gives positive advice not to publish about a dozen times a year. More often, editors err on the side of caution and omit dubious information without consulting anyone.

On a whim, I telephoned Whitehall in search of information about the D Notice system. I was put through almost at once to Bill Higgins himself. When I told him that I was a Politics teacher he replied jocularly, Well, I'm sure I can safely assume that none of your students are anarchist subversives. I thought of the two who had been arrested just the previous week for staging a sit-down demo in the middle of Oxford Street, and maintained a discreet silence. He was, incidentally, very open and informative. A day later the post brought a list of the D Notices currently in force, together with an explanatory handout.

There is an annual review of the D Notices, which took place in October and made some minor adjustments in the light of John Major's professed commitment to more open government: for example, they incorporated the public acknowledgement of MI5 and MI6, the names of their chiefs and location of their headquarters. A discussion group of media representatives (chaired by Guardian editor Peter Preston) was held just prior to that review, to ponder the whole existence of the D Notice system. There were isolated calls for the system to be abolished and replaced by separate lists of sensitive items and areas to be issued by each government department. However, it was generally felt that this would be, at least, a recipe for chaos and, at worst, would result in more rather than less secrecy. The D Notice system therefore lives on, but is of declining importance — likely to wither on the vine, as Higgins put it — in comparison to the whole panoply of Britain's secret state. For civil libertarians, the main targets of attack must still be the Official Secrets Act on the one hand and, on the other, those editors and producers who supinely collude in the withholding of information from the public even when national security is clearly not threatened. As Ernest Bevin once said in a Cabinet debate on media censorship, "Why bother to muzzle sheep?"