Humberside Police unlawfully interfered with a man's right to freedom of expression by turning up at his place of work over his allegedly "transphobic" tweets, the High Court has ruled.
Former police officer Harry Miller, 54, who founded the campaign group Fair Cop, said the police's actions had a "substantial chilling effect" on his right to free speech.
Mr Miller, who is from Lincolnshire, claims an officer told him that he had not committed a crime, but that his tweeting was being recorded as a "hate incident".
The College of Policing's 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".
In a ruling on Friday, the High Court in London found Humberside Police's actions were a "disproportionate interference" with Mr Miller's right to freedom of expression.
But Mr Justice Julian Knowles rejected a wider challenge to the lawfulness of the College of Police guidance, ruling that it "serves legitimate purposes and is not disproportionate".
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."
At a hearing in November, Mr Miller's barrister Ian Wise QC said his client was "deeply concerned" about proposed reforms to the law on gender recognition and had used Twitter to "engage in debate about transgender issues".
He argued that Humberside Police, following the College of Policing's guidance, had sought to "dissuade him (Mr Miller) from expressing himself on such issues in the future", which he said was "contrary to his fundamental right to freedom of expression".
The judge said Mr Miller strongly denies being prejudiced against transgender people, and regards himself as taking part in the "ongoing debate" about reform of the Gender Recognition Act 2004, which the Government consulted on in 2018.
Speaking outside the Royal Courts of Justice after the ruling, Mr Miller said: "Mr Justice Knowles was very clear - we have never had a Gestapo or a Stasi in Great Britain.
"Well, the actions of Humberside Police came way too close for comfort. This is a watershed moment for liberty: the police were wrong to visit my workplace, wrong to 'check my thinking'."
Holding a copy of George Orwell's Nineteen Eighty-Four, he added: "I'm going to continue tweeting, I'm going to continue campaigning and I'm going to continue standing with women in order to secure their sex-based rights.
"This judgment today has told us that we can do that and, if the police come knocking, say: 'Miller v Humberside Police, bugger off!'"
In a statement, Mr Miller's solicitor Paul Conrathe from Sinclairslaw said: "We welcome today's judgment, which is a vindication of Mr Miller's actions in posting tweets that were critical of transgender ideology and practice.
"It is a strong warning to local police forces not to interfere with people's free speech rights on matters of significant controversy.
"Today, the judge held that it is entirely acceptable to hold the view and communicate that a trans woman is not a woman. That view is not hateful, transphobic or unlawful."
A statement by Humberside Police said: "We accept the Judge’s decision today and his findings in the Judicial Review.
"The mere recording of the incident by Humberside Police as a hate incident has been ruled as not unlawful and in accordance with the College of Policing (CoP) guidance.
"Our actions in handling the incident were carried out in good faith but we note the comments of the Judge and we will take learning from this incident moving forward.
"The CoP guidance is currently being revised and will hopefully offer further clarity to Forces as to the handling of hate incidents.
"We will always take hate related offences and reports seriously as they can cause extreme distress to victims and communities, some of whom are incredibly vulnerable members of our society, and people should not suffer in silence."
The judge has granted Mr Miller permission to appeal against his ruling on the lawfulness of the College of Police's guidance.
Ian Wise QC, representing Mr Miller, asked the court to grant a "leapfrog" certificate to allow the case to go straight to the Supreme Court.
Jonathan Auburn, for the College of Police, agreed that the case was suitable to go directly to the UK's highest court.
Mr Justice Knowles granted permission for the case to "leapfrog" to the Supreme Court, subject to that court's permission.