The world of Tinker Tailor Soldier Spy is also the lost, deluded world of official secrets. Go back two or three decades to find some of the great government debacles of their day.

Remember Harriet Harman and Patricia Hewitt, when campaigners, not politicians, taking MI5 triumphantly to Strasbourg for deeming their National Council for Civil Liberties "subversive" and hacking into its telephones. Remember that ludicrous legal scurry across the globe trying to shut down reporting of Peter Wright's Spycatcher memoirs. Remember the commonsense jury which sank Mrs Thatcher, not Clive Ponting, over the Belgrano. If there were official secrets to be kept, it seemed, there was also official paranoia, miscalculation and mess. And if the Official Secrets Act of 1911 was involved, it was a blunderbuss that repeatedly blew up in Whitehall's face.

The 1989 idea of reforming the act was to give it Armalite status. Henceforth, not everything would be labelled "secret" unless cleared for specific release. Henceforth, the targeting would supposedly be far narrower, far more sentient. But still, time and again, it has proved unfair and unfit for purpose. The whole concept of an Official Secrets Act invites public disdain and increasing confusion as it sits alongside the Human Rights Act and Freedom of Information.

So how on earth does a new commissioner of the Metropolitan Police come to conclude that a superintendent in his professional standards unit is right and that the Guardian should be pursued via a bizarre combination of sections 4 and 5 in the 1989 version and required to disclose its sources for the Milly Dowler story in its pursuit of News of the World phone-hacking? Even Mrs Thatcher might have reached for a cold towel over such foolishness.

Of course, the paper's pursuit of the hackers manifestly remains in the public interest. Everyone from the last Met commissioner to the secretary of state most affected, Jeremy Hunt, agrees. David Cameron and Ed Miliband make common cause. Without the Guardian, and in particular without its revelations about the cruel hacking of a murdered schoolgirl's mobile, the Operation Weeting investigation into past, moribund investigations might still be grinding obscurely along.

The issues beyond this extraordinary Yard move couldn't be starker. Journalists have a duty to defend their sources. The Human Rights Act (and Strasbourg beyond it) makes that clear. The use of the Official Secrets Act, which lacks any public interest defence, is imaginative going on ludicrous; just as its prospective use against police officers on the Weeting team is misjudged. In effect, Scotland Yard, which muffed the scandal first time round, appears bent on muffing the task one more time in a welter of hyperactive zeal.

At least there are several more hurdles this weird initiative must yet clear before its catches fire: an Old Bailey hearing on Friday, decisions from the attorney general and Crown Prosecution Service further down the line. But even at first glance, commissioner Hogan-Howe must see the quicksands he's stumbling into, pursuing newspaper sources where the public isn't just interested, but incensed. Tinker Tailor Soldier and Plainly Intelligent Copper? Let us hope so, for everyone's sake – including the Yard's.

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