Mr Justice Eady refused an injunction to prevent The Times identifying serving officer “Night Jack”, winner of an Orwell prize for blogging.
The judge said said blogging was “essentially a public rather than a private activity”.
Night Jack’s lawyer said preserving his anonymity was in the public interest.
Hugh Tomlinson QC, said the thousands who communicated via the internet under a cloak of anonymity would be “horrified” to think the law would do nothing to protect their identities if someone carried out the necessary detective work to unmask them.
But the judge ruled any right of privacy on the part of the blogger would be likely to be outweighed by a countervailing public interest in revealing that a particular police officer had been making such contributions.
He added: “Those who wish to hold forth to the public by this means often take steps to disguise their authorship, but it is in my judgement a significantly further step to argue, if others are able to deduce their identity, that they should be restrained by law from revealing it.”
The blogger expressed strong opinions on matters of political controversy and had criticised a number of ministers.
The judge said that it had always been apparent that if his employing police authority became aware – as it now had – that one of its officers was communicating to the public in such a way, there would be a significant risk of disciplinary action.
Indeed, this was one of the main reasons why Night Jack was keen to maintain his anonymity.
Rejecting the argument that all the blogger’s readers needed to know was that he was a serving police officer, the judge said that it was often useful, in assessing the value of an opinion or argument, to know its source.
“For so long as there is anonymity, it would obviously be difficult to make any such assessment.
“More generally, when making a judgment as to the value of comments made about police affairs by ‘insiders’, it may sometimes help to know how experienced or senior the commentator is.”
He did not accept that it was part of the court’s function to protect police officers who were, or thought they might be, acting in breach of police discipline regulations from coming to the attention of their superiors.
The public was entitled to know how police officers behaved and the newspaper’s readers were entitled to come to their own conclusions about whether it was desirable for officers to communicate such matters publicly.