Thousands of bloggers churn out opinions daily — secure in the protection afforded to them by the cloak of anonymity.

From today, however, they can no longer be sure that their identity can be kept secret, after a landmark ruling by Mr Justice Eady.

The judge, who is known for establishing case law with his judgments on privacy, has struck a blow in favour of openness, ruling that blogging is “essentially a public rather than a private activity”. In the first case of its kind, Mr Justice Eady ruled that a serving police officer could not have an injunction to stop The Times identifying him as the author of the NightJack blog.

The judge applied a two-fold test that is now established in privacy cases: first, whether the police officer had a reasonable expectation of privacy (in this case over his identity); and if so, whether that right to privacy was overridden by public interest in disclosure of his name.

The police officer failed on both grounds. Just because he wished to remain anonymous, the judge said, did not mean that he had a reasonable expectation to remain anonymous; nor that The Times was under an legally enforceable duty to protect his identity.

Key to the judge’s decision was the unmasking of the News of the World undercover journalist, Mazher Mahmood, when the MP George Galloway published a picture of him on the internet in his guise as “fake sheikh”.

Mr Justice Eady was persuaded by the judge’s reasoning in that case in 2006 that a journalist who writes under a pseudonym as part of his undercover work had no reasonable expectation of privacy over his identity.

Here, the police officer was not a journalist, Mr Justice Eady said, but the function he performed via his blog was similar.

He added that people who “wish to hold forth” to the public often took steps to disguise their authorship. But it was a “significantly further step” to argue that if people could deduce their identity, they should be restrained in law from revealing it.

The judge then goes on to say that even if the officer had won the argument for a reasonable expectation of privacy, he would lose on public interest grounds.

He was a serving police constable and his work mostly dealt with police work and social and political issues relating to the police and administration of justice, on which he expresses “strong opinions”, including some on subjects of political controversy.

One reason he was keen to preserve his anonymity was fear of disciplinary action if he were exposed.