The police response to an ex-officer's allegedly transphobic tweets was unlawful, the High Court has ruled.
Harry Miller, from Lincolnshire, was contacted by Humberside Police in January last year .
He was told he had not committed a crime, but it would be recorded as a non-crime "hate incident".
The court found the force's actions were a "disproportionate interference" on his right to freedom of expression.
Mr Miller's solicitor Paul Conrathe said: "It is a strong warning to local police forces not to interfere with people's free speech rights on matters of significant controversy."
Mr Miller, 54, also launched a wider challenge against the lawfulness of College of Policing guidelines on hate crimes, which was rejected.
These define 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".
Mr Justice Julian Knowles rejected Mr Miller's challenge against the guidelines, ruling they "serve legitimate purposes and [are] not disproportionate".
Mr Miller posted a number of tweets between November 2018 and January 2019 about transgender issues as part of the debate about reforming the Gender Recognition Act 2004.
'Participant in public debate'
In one tweet Mr Miller wrote: "I was assigned mammal at birth, but my orientation is fish. Don't mis-species me."
This tweet among several others which were reported to Humberside Police as being allegedly transphobic.
Officers visited Mr Miller's workplace and in a statement said they had "received reports of a number of transphobic comments being posted on social media".
Mr Miller's barrister, Ian Wise QC, argued the force's response had sought to "dissuade him from expressing himself on such issues in the future".
Mr Justice Knowles said Mr Miller "strongly denies being prejudiced against transgender people" and had regarded himself as a participant in a public debate.
The judge said: "The claimants' tweets were lawful and there was not the slightest risk that he would commit a criminal offence by continuing to tweet.
"I find the combination of the police visiting the claimant's place of work, and their subsequent statements in relation to the possibility of prosecution, were a disproportionate interference with the claimant's right to freedom of expression because of their potential chilling effect."
Mr Miller has appealed against the ruling about the College of Policing guidance and permission has been granted for the case to go straight to the Supreme Court.
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