As no one transgressed and the exact identity of those named remained a secret (except to anyone with the tenacity to find the site on their own), it seemed that all was still working well. But in the age of new communications - which are increasingly outside the control of any government or media group - the affair also raised inevitable questions about whether this relic of the Imperial era is now either effective or desirable.
The issue can also be expected to form a significant part of the debate which will surround the Government's eventual move towards creating a freedom of information act. Once most areas of state information are regulated by legislation, can such an anachronistic arrangement over its most sensitive secrets still be left to a quiet word in an editor's ear?
Given that this avuncular advice is already backed by the Official Secrets Act and, in the M16 case, a specific court order, is its supposed voluntary nature anything more than a cosy illusion anyway?
"The whole concept is obviously outdated," says Roy Greenslade, the media commentator and former editor of the Daily Mirror. "The idea that there is something called the `national security', that mustn't be violated, is a Second World War going into a Cold War concept. Post-Cold War, it makes no sense at all. What is also remarkable about D-Notices is that editors still obey them."
While a survivor from the end of the Edwardian period, the Defence, Press and Broadcasting Advisory Committee (to give it its full title) can be seen as a response to the first very modern concerns about state secrecy. It came in the aftermath of the second Official Secrets Act of 1911, itself designed to plug leaks from a new, lowly class of public employees, who were as likely to have come through the trade union movement as public schools and Oxbridge, and so were deemed to be lacking in the culture of honourable secrecy. The act, which survived unchanged until "reformed" by the last government, covered all holders of public information and all unauthorised communications, irrespective of public interest.
In terms of the press, however, it was clearly thought that the right sort of chap was still in charge, and the idea of a voluntary code was formulated by the then director of naval intelligence. His idea was to "Put the press to their honour, in the schoolboy sense of the term... by issuing a communique to the press association stating what is to be carried out, and asking them to cooperate in [foregoing] the publication of information likely to be of value to foreign countries." Which was how the D-Notice Committee came to be formed in 1912.
It currently includes representatives from the Newspaper Publishers Association, the Periodical Publishers Association, the Scottish Daily Newspaper Society, ITV and Sky TV, as well as officials from the Ministry of Defence (MoD), the Home Office, the Foreign and Commonwealth Office and the Home Office. It only meets twice a year, but has a full-time operation run from Room 2235 of the MoD's main building in Whitehall and overseen by its secretary, Rear Admiral David Pulvertaft.
It is Admiral Pulvertaft who is contacted by concerned government departments if a potentially sensitive story comes to light, and it is he who makes the friendly calls to editors warning them that they may be about to breach the gentleman's agreement.
Last week, after the MI6 list had been discovered on the Internet, he issued an advisory notice to the effect that none of the names should be published, and warning that to do so could "put lives at risk". He added that there was concern that the list may have been put out by a former member of the Security Intelligence Service (MI6).
All Defence Advisory Notices (DA's) fall under one of six headings agreed by the Committee, covering everything from operational plans to nuclear weapons, secure codes and the identification of specific defence sites. Last week's notice came under DA No 6, which covers United Kingdom Security and Intelligence Services.
Moves already made by Government lawyers, however, put legal muscle behind the request, suggesting there was concern that, on its own, the voluntary system might not be enough. The clear understanding on all sides was that the list had been put on the Internet by Richard Tomlinson, the former MI6 officer who was sacked and subsequently served six months in prison for breaching the Official Secrets Act. He now lives in Geneva, and two weeks ago the lawyers took out a court injunction in Switzerland preventing him from disclosing any information gleaned from his past employment, including through the Internet. An existing High Court order from November 1996 prevents the British media from publishing any information received from Tomlinson.
So, faced with prosecution of themselves and the owners of their organisations, editors had no choice but to comply. So much for being placed on their schoolboy honour.
The former Tory MP Rupert Allason, who writes spy books under the name Nigel West, was asked to delete some names from his book The Crown Jewels, which used documents that had been released in Moscow. "It was a slightly silly problem, but I took it in good grace," Allason says.
A more forceful recent example of the D-Notice Committee's activity is the case of author Tony Geraghty. Last week he was charged under section 5 of the Official Secrets Act over his book, The Irish War, published earlier this year. The alleged breach of security comes in a section of the book relating to military intelligence computers used for surveillance in Northern Ireland.
The important point here is that Mr Geraghty, 61, was approached by Admiral Pulvertaft last year while he was still writing the book, when the Admiral expressed concerns over some of Geraghty's sources. The author's obvious in-depth knowledge of military surveillance and counter-surveillance techniques, said the Admiral, "have dangerous implications for the population at large." But Geraghty declined to cooperate, and says the Admiral responded by hoping he would not come to regret this.
On 3 December last year, Geraghty's home in Herefordshire was subjected to a dawn raid by MoD police, during which they siezed his computer, its files, and other documentation. Geraghty was arrested and interrogated for five hours. Admiral Pulvertaft has since denied any collusion between himself and the MoD police.
Some observers still remain sceptical, and say that if the D-Notice system has covertly changed from being voluntary to being coercive, then the time may have come to scrap it altogether.
"It is possible to make a case that the system has worked reasonably well in the past," says Stephen Glover, media commentator for The Spectator and former editor of The Independent on Sunday. "But if it is going to be manipulated for political ends, then perhaps it would be better if it now came under legislation. The danger is, of course, that having it put into law might be a lot more coercive than what we have already."
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