The Home Secretary, Jack Straw, says MI5 agents could die if the former MI5 officer David Shayler is allowed to disclose evidence which he claims demonstrates the incompetence of the Security Service.
Mr Straw says he has an "absolute duty to ensure agents' lives" are not put at risk. That is the basis on which he has successfully sought an injunction against The Mail on Sunday whose editor wants to publish "the facts of a case of national and international importance which would have revealed MI5's incompetence in the handling of a serious terrorist incident".
Mr Straw would have us believe that he'd agonised over so grave a step as censorship - "I have no wish to prevent legitimate debate or criticism of the Security Service" - no doubt mindful of his government's commitment to freedom of information.
"Censorship" is certainly a word that should rankle with Mr Straw. He was himself a journalist on Granada's World In Action programme. And in 1971, as a duffel-coated President of the National Union of Students, he petitioned the then Conservative Home Secretary, Reggie Maudling, against the expulsion of a German Marxist student leader Rudi Dutschke whose presence was said to be a threat to the security of the state. As Mr Shayler revealed, in those days M15 had Mr Straw down as a "communist sympathiser".
But that was then. Now as Home Secretary he is ultimately responsible for MI5. And if the MI5 Director General tells him the publication of a newspaper article could cost lives, he has to take that seriously - even if the risk is theoretical.
Why, then, in another case involving a real live agent, whose risk of being executed by the IRA is very high indeed, has the Home Secretary been playing down the threat to his life?
Martin McGartland spied for the RUC Special Branch and saved many lives. As the late Brian Fitsimons, former assistant chief constable in charge of the Special Branch, told me: "McGartland was a very productive agent."
Since 1991 the 27 year old has lived in Blyth, Northumbria as "Martin Ashe" - the identity provided for him by the RUC. Last May he was tried at Newcastle Crown Court for perverting the course of justice because he had two driving licences in the name of "Ashe" which listed two different addresses.
The police said he was trying to avoid disqualification by distributing penalty points for speeding between the two licences. McGartland denied his, insisting he needed the licences as an extra layer of protection.
In 1992 the IRA had interrogated his girlfriend and he feared they'd forced her to divulge where he was living. He says the police refused to move him: "I'll never forget what they told me:'Don't worry about it, son. You'll be OK.'" He didn't believe them. So he moved - though only half a mile away. And he took out a second licence with a fictitious address in Durham.
McGartland feared the IRA could get his address from licence records - and with good reason: Northumbria Police has already dismissed one civilian for disclosing information about his identity from their computer. The purpose of the second licence was to send the IRA off on a false trail if they came looking for him having discovered he no longer lived at his first address.
There is no doubt that McGartland was on the IRA's wanted listed. They'd broken his younger brother Joseph's legs, an arm and four ribs with iron bars. And he himself received a mass card, signed "Your friends in Connolly House [Belfast HQ of Sinn Fein], Crumlin Road and Long Kesh." It promised that the "Holy Sacrifice of the Mass will be offered for the repose of the soul of Marty McGartland."
Before the trial McGartland warned the Crown Prosecution Service that the only way he could escape a prison sentence was to explain to the jury who he really was and why he'd taken out two licences - which involved disclosing who he really was.
The jury took 10 minutes to acquit him and even the judge said McGartland did not seem to be a criminal. But his cover was blown McGartland's real identity and address were reported in the newspapers.
McGartland asked the authorities to give him a new identity and relocate him immediately. One thousand nine hundred local residents - astonished to discover their chaotic Irish neighbour was a latter-day 007 - signed a petition supporting him. Close neighbours have been frequently wakened by his screaming nightmares: "Don't do it, don't do it." Some neighbours are pretty terrified themselves. One is having her locks changed by Northumbria Police.
At first the Crown refused to pay McGartland a penny. After many letters from his lawyer to the solicitors acting for "various Crown authorities" - which they refused to name - they agreed to pay pounds 3,000 towards his removal expenses "as a gesture of goodwill and with no admission of liability".
But McGartland's house is now blighted. Who would want to buy a property that was an IRA target? Until recently, the "authorities" have refused to even consider covering any losses. Jack Straw has told McGartland's local MP, Alan Campbell, that there is no need for him to move straightaway because there is no "immediate" threat to him. "I hope this is of some reassurance," says Mr Straw.
The small sum now being offered contrasts with the millions of pounds that McGartland's tip-offs spared the taxpayer by stopping buildings from being blown up - not to mention the many lives he saved. McGartland was also very nearly executed by the IRA because of an RUC blunder in August 1991.
For several months he had complained to his Special Branch handlers that he feared the IRA had rumbled him. His active service unit had had to abort too many operations in which he'd been involved.
Then in July 1991, McGartland's handlers learnt that his ASU was going to machine-gun a bar frequented by soldiers in Bangor. They demanded that McGartland deliver the guns that were to be used to RUC headquarters to be fitted with tracking devices.
Two IRA operatives were arrested en route to the raid. A few days later McGartland was summoned to a meeting with the head of the IRA's internal security. Although McGartland feared the game was up, his handlers persuaded him to keep his appointment, promising they would watch over him every step of the way. The Special Branch had calculated there could be a bonus if he was caught. He could lead them to other IRA terrorists as they interrogated him. He was to be used as human bait.
But the RUC's surveillance unit lost sight of McGartland before he was driven away at speed by two IRA men from his meeting. As one officer said: "I thought `Well, it's over. He's gone.' And I waited for news of his body being found."
McGartland, meanwhile, was awaiting the executioner's bullet in a third- floor flat. When he saw a bath filled up with cold water, he knew he would be tortured first. On an impulse he hurled himself headlong through the window. "I remember the glass breaking in slow motion. Then my lights went out." He sustained serious head, shoulder and back injuries, but his astonishing courage saved him. He was picked up by an army patrol.
Six weeks later he was packed off to Northumbria with a new identity. The police bought him a house for pounds 53,500 and gave him pounds 40,000 to provide him with furniture, a car, spending money and a training course for a heavy goods vehicle licence.
But McGartland had paid a heavy price. He'd left behind his family and had no friends in Newcastle. He suffered recurring nightmares, needed constant painkillers, and he became depressed. He couldn't settle into a job and he frittered away his money But the RUC who'd recruited him at 17, had cut the umbilical chord.
Like many agents McGartland wanted recognition. He tried to sell his story and began to appear on TV programmes and to give newspaper interviews. This can't have gone down too well with the authorities.
When his solicitor applied for compensation for his injuries, the RUC claimed to have no record of him. He pursued the claim in the courts. But when he asked the RUC for protection in Northern Ireland so he could attend his court case, they refused. "Any protection that was afforded you, you blew by your antics," one officer told him.
Then the RUC told the Northern Ireland Office they'd already paid McGartland pounds 120,000 which, they said, included injury compensation. His receipts showed the figure was pounds 95,000, a shortfall of pounds 25,000. Only after Tory, Labour and Unionist MPs intervened, did the NIO compromise with an offer of pounds 10,000.
Earlier this year, McGartland published a book detailing his exploits. At the same time the CPS was considering whether to prosecute him for perverting the course of justice. After consulting the RUC, it was decided there was no threat to his security. Listening to some RUC officer discussing this issue, there's a strong whiff of disdain: McGartland was recruited as a teenage petty criminal and in his attempts to hog the limelight he'd moved above his station. It's hardly surprising he came a cropper.
No doubt, running McGartland as an agent was taxing. Like most agents, he sometimes broke the rules by not telling his handlers everything in order to cover his back with the IRA. Sometimes he bartered information for money. But, as one ex-RUC officer recognises, "I would have to say that we got more out of him in a few years than we got out of many agents in 20 years. It was short - but very sweet while it lasted."
McGartland was very young to be drawn into the grown-up world of spying. By 21, he had saved many lives. But now he doesn't know how to save himself. His obsession with his string of grievances and his craving for recognition leave him incapable of building a more stable future. What burden of responsibility should the Crown now bear for trying to ensue that the rest of his life is not wasted?
Not much, seems to be the answer. As the new Home Secretary, perhaps still seeking the approval of the security and intelligence establishment that once had him down as soft and on the left, Jack speaks of this as being "very much a security matter". But it is also a human tragedy.
In his letter to McGartland's MP, Alan Campbell, Jack Straw suggests that it is his "propensity for self-publicity that has caused [him] so many of the problems he currently faces".
Mr Straw misses the point. McGartland may have raised his profile but he never once revealed that he was "Martin Ashe" in any article or TV programme. It was the Crown's decision to prosecute him that led to that. Now that he has been acquitted, he claims he is entitled to the same level of assistance a he was before the court case invalidated his protection.
The McGartland case also raises a question about double standards: in Newcastle an ex-Special Branch agent whose life is manifestly at risk is deemed by the Home Secretary not to be in any "immediate" danger. In London, a risk, perhaps theoretical, to the lives of MI5 agents is enough for him to ban publication of a matter of public interest about a secret and largely unaccountable organisation.
The one feature common to both cases is publicity. Do the intelligence services still cling to the belief that it is dangerous for light to be shed on their workings. If so, why does New Labour swallow old myths?
The story of Martin McGartland will feature in `Here and Now' , BBC1, 7.30pm tonight.