A seven-year-old boy who was "living life entirely as a girl" has been removed from his mother's care following a ruling by a High Court judge.
Mr Justice Hayden said the woman had caused her son "significant emotional harm".
The judge said the woman had been "absolutely convinced" that the youngster "perceived himself as a girl" and was determined that he should be a girl.
Detail of the case emerged on Friday in a ruling by the judge following a hearing in the Family Division of the High Court in London.
Mr Justice Hayden said no-one involved in the case could be identified.
The judge said the boy was now living with his father, who is separated from the mother.
Mr Justice Hayden said he had analysed evidence from the boy's parents, local authority social workers and a psychologist.
He indicated that the boy's parents had separated some years ago.
The boy had stayed with his mother.
Family court litigation had started about three years ago after the father raised concerns about not having contact with his son.
A lower-ranking judge had authorised a "wide-ranging" inquiry and local authority social services staff had begun investigations.
"(His mother) told me that (he) was 'living in stealth' by which was meant, she explained, that he was living life entirely as a girl," said Mr Justice Hayden in the written ruling.
"He dressed, at all times, like a girl and, it transpired, had been registered at a new general practitioner's as a girl."
The judge added: "I was also left in no doubt that (the mother) was absolutely convinced that (the boy) perceived himself as a girl."
Mr Justice Hayden said his "overwhelming impression" was that the woman "believes herself to be to fighting for (her son's) right to express himself as a girl".
He said the woman had told him how the boy "expressed disdain for his penis".
The judge added: "I consider that (the mother) has caused significant emotional harm to (her son) in her active determination that he should be a girl."
Mr Justice Hayden said the boy's father was shocked when the youngster moved in with him and his partner.
But he said the boy had settled down well in his father's care.
"I have been told that (the father) and his partner were shocked when they first saw (the boy) by the extent to which he appeared to be a girl, both in appearance and in mannerism," said the judge.
"However, what is striking is how well (the boy) has settled down."
The judge added: "I have noted from reports that (the boy) has become interested in Power Rangers, SpongeBob, Superheroes and is constantly finding new interests...
"It is striking that most of (the boy's) interests are male-oriented. I am entirely satisfied, both on the basis of the reports and (the father's) evidence at this hearing, that he has brought no pressure on (the boy) to pursue masculine interests. (The boy's) interests and energy are entirely self-motivated."
He said the boy's father had shown himself to be a "natural and instinctive father".