The most puzzling thing about the decision to arrest Damian Green is why anyone in the Metropolitan Police thought the inquiry would lead to a successful prosecution.

The bitter end to the cash-for-peerages inquiry is still fresh in the force’s memory. Then, Scotland Yard was arresting senior Labour Party people in dawn raids on suspicion of committing much more serious offences and enduring a barrage of political criticism for its actions.

But the Yard was pretty confident of its case and ploughed on stubbornly only, in its view, to be let down when the Crown Prosecution Service refused to press charges.

The Green affair is nothing like as serious as alleged cash- for-peerages. And worse, even as the Met was planning the arrest, the offence it was relying on was being systematically demolished in the courts.

On Tuesday last week a judge at Kingston-upon-Thames Crown Court threw out a case against Sally Murrer, a journalist charged with aiding and abetting misconduct in a public office — the same charge that the Met wants to pursue against Mr Green.

The Murrer case turned on Article 10 of the European Convention on Human Rights, the right to freedom of expression. The court ruled, as courts across Europe have ruled, that leaks to journalists are not criminal unless they involve matters of national security or impair the investigation of serious crime.

The evidence against her — gained by planting bugging devices and raiding her home and her office (sound familiar?) — was ruled to have been obtained unlawfully.

It is not hard to imagine how a reasonably competent lawyer, citing the Murrer case, could put the case against Mr Green through the shredder.

What is troubling, however, is that the Met knew about the Murrer case and its significance because it had advised Thames Valley officers on how to proceed with the investigation.

Also in the loop was the CPS, which decided on Friday to drop charges against Murrer and the policeman who leaked the stories to her. The Home Office, which began the leak inquiry that netted Mr Green, also knew about Murrer.

In the light of all that recent experience, one is left wondering why anyone in their right mind thought that it was sensible to arrest Mr Green.

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