Croydon Advertiser reporter Gareth Davies did absolutely nothing wrong when he followed up a local fraudster's conviction with fresh stories about her criminal conduct. For the police to issue him with an anti-harassment “police information notice” – and for the Independent Police Complaints Commission to go along with that – is completely wrong, legally and morally.

Davies did everything that any responsible reporter would have done to stay within the law. Indeed, by contacting the fraudster he was, in fact, obeying the law. What the police have since done to him has no legal basis and sets a dangerous precedent which makes it harder for responsible journalists to abide by the law.

When a journalist writes a story he must abide by the civil law of defamation. If he wants to publish something based on a previous court verdict – in this case, the fraudster's earlier conviction – then he is under a duty to contact the criminal, put the key part of his story to her and then to publish any statement of explanation or contradiction that she gives. This duty stems from the Defamation Act 1996, schedule 1, part 2.

Davies discharged this duty impeccably. He emailed the fraudster and when she failed to reply, he knocked on her door in order to speak to her in person. This is a standard method used by journalists all over the country to ensure that story subjects cannot later claim they were not contacted or did not receive messages left for them.

The fraudster complained to police using harassment laws intended to protect victims of domestic violence from being contacted by abusive partners. The fraudster alleged that Davies had contacted her on multiple occasions, which was false.

Croydon Police did not investigate her claims or apply any test of reasonableness to them. They took them at face value, dispatched three constables to the newspaper's offices to issue the "police information notice" and threatened Davies with arrest if he continued reporting the convicted fraudster's criminal activities. When Davies quite rightly said he was doing his job, one of those constables, PC Chapman, sneered: “That’s what the News of the World phone hackers said.”

It looks to me as if Davies has the law firmly on his side. Section 1(3)(b) of the Prevention of Harassment Act 1997 (PHA) states that no course of conduct can amount to harassment if it "was pursued under any enactment or rule of law". Clearly Davies was discharging his duty under the Defamation Act and so cannot be said to have harassed the fraudster in asking her for comment.

The “police information notice” issued to him by Croydon Police has no basis in law. It is an internal procedure developed by the police without any legal authority and is not subject to any statutory oversight or control, as lawfully conferred and exercised police powers are. It consists of a formal notice that a particular course of conduct may amount to harassment, as defined in PHA. The notice is delivered to the alleged harasser by police in person and is also recorded on various police systems which may be accessed by other public agencies.

Crucially, it can be included on an enhanced criminal records bureau check, or DBS disclosure as it is now known, as part of information held on a person's criminal record. The notice is therefore likely to be taken as a statement of criminal conduct which is endorsed by the police, and not as a speculative and baseless allegation made by a criminal attempting to evade the consequences of her lawful conviction. A Parliamentary briefing paper notes that the police's stated intent is to use these notice as evidence in any later prosecution.

Davies' complaint against the notice to the Metropolitan Police was snottily dismissed by one Inspector Claire Robins, of Croydon Police, who not only took a convicted criminal's word at face value – telling Davies “the police do not have to investigate the matter” – but even took it upon herself to start dictating what reporters should and shouldn't do. His subsequent appeal to the Independent Police Complaints Commission was denied, on the bizarre and illogical grounds that as harassment notices are not issued under the PHA (because there is no statutory power for police to issue such notices), the statutory defences do not apply.

Pinch yourself: this isn't a tale from Catch-22, the Met Police really are making up specious harassment “laws”, using them to harass law-abiding journalists, and then claiming real laws passed by Parliament don't apply to police conduct.

Let's repeat that just to be crystal clear: the police have made up an internal procedure, loosely modelled on the law but with no actual basis in law at all, and used it to smear and intimidate a journalist who was lawfully doing his job. Then they deprived the journalist of his protection under the law after improperly issuing one of these made-up notices… because the police are themselves not acting in accordance with any law at all.

This is plainly intolerable behaviour in a democratic society subject to the rule of law.

By issuing a police information notice to a reporter discharging his lawful duty, Bernard Hogan-Howe's Metropolitan Police have not only erred in law, they have sided with a criminal against the media and against the public. It is high time we journalists write to our MPs and (for Londoners) Mayor of London Boris Johnson and our London Assembly members to demand this behaviour is reined in immediately. We cannot have arbitrary police action overriding the law of the land just because it suits a clique of coppers who, for some bizarre reason known only to them, prefer trusting the word of a convicted con-woman over a reporter of good character.

Perhaps this is what Hacked Off mean when they talk about “the Leveson effect”.

Gareth Corfield is chief sub editor of The Register writing in a personal capacity and has raised his concerns around the PIN issued against Gareth Davies with his London Assembly Member Victoria Borwick.