Official police guidance on recording hate incidents against transgender people imposes a “substantial chilling effect” on freedom of expression, the high court has been told by lawyers representing a former officer.

Harry Miller, who served with Humberside police, was contacted by the force this year after a complaint from a member of the public about allegedly transphobic comments he made on his Twitter account @HarryTheOwl.

Another officer told Miller that he had not committed a crime but his tweeting was being recorded as a hate incident under the College of Policing’s guidelines on hate crime, the high court in London heard.

The guidance defines a hate incident as “any non-crime incident which is perceived, by the victim or any other person, to be motivated by a hostility or prejudice against a person who is transgender or perceived to be transgender”.

Miller has launched legal action against the College of Policing, the professional body for the police service in England and Wales, and Humberside police. It is the second high-profile case being heard this week in London involving a clash between free speech and transgender rights.

Opening his claim on Wednesday, Miller’s barrister, Ian Wise QC, said his client was “deeply concerned” about proposed changes to the law on gender recognition and had used Twitter to “engage in debate about transgender issues”.

Wise argued that Humberside police, following the College of Policing’s guidance, sought to “dissuade [Miller] from expressing himself on such issues in the future”, which Wise said was “contrary to his fundamental right to freedom of expression”.

At the outset of the hearing, the judge hearing the case, Mr Justice Julian Knowles, said he was “not concerned … with the merits of transgender issues on either side of what is obviously a very hotly contested debate”, and any changes to the law were “a matter for parliament and not the court”.

Wise said Miller had been tweeting “extensively” about proposed changes to the Gender Recognition Act for a number of years. “The claimant has never expressed hatred towards the transgender community or sought to incite such hatred in others, and has simply questioned (at times provocatively and using humour) the belief that trans women are women and should be treated as such for all purposes.”

He said his views “form part of a legitimate public debate and cannot sensibly be regarded as hate speech”, and it would be “a classic case of a ‘heckler’s veto’” if it was permissible to interfere with his right to freedom of expression because his tweets “caused offence to some readers”.

In written submissions, Jonathan Auburn, representing the College of Policing, said Miller “engaged in regular tweeting relating to transgender people” and in one message said: “I was assigned mammal at birth, but my orientation is fish. Don’t mis-species me. Fuckers.”

Auburn added: “While the claimant now expressly disavows having any personal hostility or prejudice towards transgender people, his social media messages speak for themselves.”

Any transgender person reading such messages would consider them to be “motivated by a hostility or prejudice against a person who is transgender”, Auburn said. The guidance was lawful and caused no interference with Miller’s right to freedom of expression, he told the court.

Humberside police is also resisting the claim, arguing that there was no or only “minimal” interference with Miller’s freedom of expression.

In a separate case at a central London employment tribunal this week, a researcher who lost her job after tweeting that transgender women cannot change their biological sex is defending her right to express her “philosophical beliefs”.

Maya Forstater, 45, a tax expert, was a visiting fellow at the Centre for Global Development (CGD), an international thinktank that campaigns against poverty and inequality. Her contract at the organisation, which is based in Washington and London, was not renewed in March this year after a dispute over her publicising her views on social media.

She was accused of using “offensive and exclusionary” language in tweets opposing government proposals to change the Gender Recognition Act to allow people to self-identify as the opposite sex.

Forstater tweeted that “men cannot change into women”. She has argued that her “gender critical” beliefs should be protected under the 2010 Equality Act, and has funded her legal challenge through the CrowdJustice website. Judgment has been reserved.

The issue is both highly sensitive and legally complex. Opponents point out that under section 9 of the 2004 Gender Recognition Act, “where a full gender recognition certificate is issued to a person, the person’s gender becomes for all purposes the acquired gender”.

Index on Censorship is supporting both Miller and Forstater. Speaking outside court, its chief executive, Jodie Ginsberg, said: “It is … of vital importance to understand the interaction between lawful speech on social media platforms and the chilling effect from police investigations into such speech.”