Thousands of bloggers who operate behind the cloak of anonymity have no right to keep their identities secret, the High Court ruled yesterday.

In a landmark decision, Mr Justice Eady refused to grant an order to protect the anonymity of a police officer who is the author of the NightJack blog. The officer, Richard Horton, 45, a detective constable with Lancashire Constabulary, had sought an injunction to stop The Times from revealing his name.

In April Mr Horton was awarded the Orwell Prize for political writing, but the judges were unaware that he was using information about cases, some involving sex offences against children, that could be traced back to genuine prosecutions.

His blog, which gave a behind-the-scenes insight into frontline policing, included strong views on social and political issues.

The officer also criticised and ridiculed “a number of senior politicians” and advised members of the public under police investigation to “complain about every officer . . . show no respect to the legal system or anybody working in it”.

Some of the blog’s best-read sections, which on occasion attracted half a million readers a week, were anecdotes about cases on which Mr Horton had worked. The people and places were made anonymous and details changed, but they could still be traced back to real prosecutions.

In the first case dealing with the privacy of internet bloggers, the judge ruled that Mr Horton had no “reasonable expectation” to anonymity because “blogging is essentially a public rather than a private activity”.

The judge also said that even if the blogger could have claimed he had a right to anonymity, the judge would have ruled against him on public interest grounds.

The police officer, the judge said, had argued that he should not be exposed because it could put him at risk of disciplinary action for breaching regulations. But Mr Justice Eady criticised that argument as “unattractive to say the least”.

He added: “I do not accept that it is part of the court’s function to protect police officers who are, or think they may be, acting in breach of police discipline regulations from coming to the attention of their superiors.”

He added: “It would seem to be quite legitimate for the public to be told who it was who was choosing to make, in some instances quite serious criticisms of police activities and, if it be the case, that frequent infringements of police discipline regulations were taking place.”

The action arose after Patrick Foster, a Times journalist, identified the NightJack blogger “by a process of deduction and detective work, mainly using information on the internet,” the judge said.

Hugh Tomlinson, QC, for Mr Horton, had argued that “thousands of regular bloggers . . . would be horrified to think that the law would do nothing to protect their anonymity if someone carried out the necessary detective work and sought to unmask them”. Mr Tomlinson said that Mr Horton wished to remain anonymous and had taken steps to preserve his anonymity.

But Mr Justice Eady said that the mere fact that the blogger wanted to remain anonymous did not mean that he had a “reasonable expectation” of doing so or that The Times was under an enforceable obligation to him to maintain that anonymity.

Antony White, QC, for The Times, argued that there was a public interest in non-compliance by a police officer with his obligations under the statutory code governing police behaviour and also with general public law duty on police officers not to reveal information obtained in the course of a police investigation other than for performing his public duties.

Lancashire Constabulary said: “He has been spoken to regarding his professional behaviour and, in line with disciplinary procedures, has been issued with a written warning.”