On the whole, and in the main, and everything considered, you do not in a democracy go around arresting the Opposition. For some time now, web humorists have been spelling new Labour “Nu-Labour”. As reports of Damian Green's arrest swirled yesterday, the prefix ZA attached itself to the bloggers' joke: ZANU-Labour. If by lunch I had heard the comparison with Zimbabwe once, I had heard it a dozen times.

Nine - nine - counter-terrorism officers? Raids (for that is what we would call them in Russia) on the home and offices of a senior member of the Opposition? What a blunder. What an outrage. What a stupid, stupid, thing to do. The best argument for doubting that ministers had anything to do with the arrest of a mild-mannered and distinctly herbivorous Shadow Immigration Minister is that this is a gift to the Tories, and incredibly damaging to a governing party whose Prime Minister enjoys a reputation for bullying.

Maybe ministers really were kept in total ignorance, but few ordinary voters are going to believe it. A Prime Minister otherwise known as the Big, Clunking Fist will struggle to dissociate himself in the public mind from an astonishingly heavy-handed police operation against a critic.

For me, Thomas à Becket and Canterbury Cathedral spring to mind. I picture an infuriated Prime Minister bellowing at a flat-screen television: “Will nobody rid me of these troublesome leaks?” Who the four knights were who took it upon themselves to act upon the presumed wishes of a maddened monarch, we may never know, but when Mr Brown insists that he didn't actually know, it is possible to believe him.

He certainly should have known. So should the Commons Speaker. If Michael Martin did have advance knowledge of a raid on a Privy Councillor's offices within the Palace of Westminster by anti-terrorist officers investigating activity that most MPs would regard as part of their job, and did know there was not the remotest suggestion that national security or any serious crime of any sort was involved, and the supposed offence amounted to nothing more than embarrassing ministers with information they had been trying to hide, then there arise some serious questions about his position. Mr Martin is an MP. He, above all others in our unwritten constitution, is there to protect the status and interests of Parliament.

This is not a small matter. It goes to the heart of parliamentary privilege. It is surely common sense that MPs' Commons offices are not to be raided by the State except for the gravest of reasons; that Mr Speaker should be urgently consulted (not “told”) in advance. Can it really be that Mr Martin simply nodded this through? Did he not ask his clerks? If he did, what was their advice? Did he not speak to the Home Secretary? She claims nobody did.

It would be comforting to think that there will be widespread disquiet on Labour's backbenches as well as among the Opposition. I hope so. Some of Mr Brown's colleagues will remember (Westminster lobby correspondents certainly do) that he made his career as Shadow Chancellor by publishing leaked documents from the Treasury.

In this long series of misjudgments, the first and biggest has been that of the outgoing Metropolitan Police Commissioner, Sir Ian Blair. If he thought - as some voices have suggested - to take a parting shot at a Conservative Party that, through Boris Johnson, has effectively removed him, then the tactic backfired. It is Labour ministers who have been embarrassed.

The decision by the head of the Home Office, Sir David Normington, to call in the police to help him plug a persistent leak from his department, is defensible - though he should have reflected that the police cannot be turned on and off like a tap and used as a kind of private detective agency. Instant dismissal is a sufficient sanction against civil servants who leak, and mandarins with long memories will recall that what looked like the Tories' vengeful pursuit of Sarah Tisdall and Clive Ponting through the courts did nobody any good. If, after the arrest of Mr Green's alleged source, senior civil servants realised where the police inquiry was headed next, it is not yet clear. If they did, it is hard to see why ministers were not informed.

I cannot avoid the suspicion that decisions were taken in Whitehall against the background of an enraged Prime Minister storming around and demanding the heads of leakers on plates. Commentators seeking method in this madness suggest that there was a stratagem: to put the frighteners on other moles, especially the sources of deeply sensitive Treasury leaks. Again, if so, the strategy has backfired. Any deterrent effect on Whitehall moles has been vastly outweighed by the political cost.

Two defences to the thinking that led to this mess need to be answered. The first is that those who offer, and those who receive, information that ministers are trying to hide may indeed be breaking the law. That is true. But the argument is too strong.

The common law offence of “aiding and abetting, counselling or procuring misconduct in a public office” sets such a ridiculously low hurdle that thousands of my colleagues in the newspaper industry, many MPs, most Opposition spokesmen, and innumerable helpfully indiscreet police officers would be behind bars if every offence was investigated and prosecuted. Much journalism would become impossible, legitimate questioning and debate by MPs would be ruled out, and activity in the public interest would be outlawed. So (as the dismissal of the case against Sally Murrer, reported in The Times today, shows) this law needs to be handled with extreme discretion. In Mr Green's case it has not been.

The second defence is that ministers are damned if they do and damned if they don't interfere. What if the Home Secretary had been warned by Mr Speaker about the raids, and pressured the Met behind the scenes to call the whole thing off? In the recent loans-for-honours affair, attempts by ministers to discourage police inquiries would have been regarded by the media and the Opposition as disgraceful.

This argument has force, but should be answered by the observation that there are ways and ways of letting chief constables know ministers' minds; that it's not always wrong to do so; but that particular care needs to be taken where the suspicion might arise that ministers are protecting their own political interests. That was not the case here, and I doubt many people would have thought the Home Secretary wrong to let Sir Ian know that, though she had no power to stop him, she was alarmed at his officers' plans.

Who now knows where this will end? Insomniacs may have heard it first on the World Service of the BBC, and could have been forgiven for assuming the story came from some benighted Central Asian republic. That it is about our own country is shaming. This will end up damaging almost everyone it touches - except the bewildered immediate victim of the fiasco: Damian Green.

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