A five-year-old may be the UK’s youngest transgender person recognised by the NHS, with parents who say they will “leave it up to him to decide what he wants to do”.

Born Zach Avery but now living as a girl, the Daily Telegraph reports that he was diagnosed by the NHS as having Gender Identity Disorder at age four.

Mother Theresa Avery said: “He just turned round to me one day when he was three and said: ‘Mummy, I’m a girl’. I assumed he was just going through a phase and just left it at that.

“But then it got serious and he would become upset if anyone referred to him as a boy.

“He used to cry and try to cut off his willy out of frustration.”

Zachy, as he is now known, was taken to doctors who diagnosed the condition.

Theresa added that Zachy’s school had been supportive, saying: “We explained to the other kids at the school that Zachy’s body was that of a boy but in his brain he was a girl. We said Zach was just happier being a girl than a boy.

“But the other kids haven’t batted an eyelid, they’ve accepted Zach as Zach and there’s been no problems at the school with bullying.”

The primary school had also made the toilets unisex for his age group.

Continuing to refer to Zachy using male pronouns, she said: “He just wants to be like a little girl and he’s very happy with his long blonde hair, pink and red bedroom and a wardrobe full of girls clothes.

“He likes playing with his sister’s old toys but he still loves Dr Who too and playing with his brother. And we still put some neutral clothes in his wardrobe if he ever decides he wants to wear them.

“We leave it up to him to decide what he wants to do – if he changes his mind and wants to be a boy again then he does, but if he doesn’t, he doesn’t.

“I would love to have my son back, but I want him to be happy. If this is the route he wants to take – if this is what makes him happy – then so be it. I would rather him have my full support.

“People need to be aware of this condition because it’s very common but even many family support workers have never heard of cases in children. There are people out there but they don’t want to talk about it.”

A spokesperson from the Tavistock and Portman NHS Foundation Trust told the Telegraph: “The diagnosis of GID is made by the key workers working with the young person. We will also assess their general wellbeing. We remain in contact with young people often for many years.

“Our aim is not to predict or direct the outcome, but rather to support the young person in their general development as well as develop a trusting collaborative therapeutic relationship in which it is possible to openly explore their feelings about their gender.”

The Trust told the paper 165 children have been diagnosed with gender dysphoria this year and only seven children under the age of 5 were diagnosed last year.

Last week, a blogger provoked widespread debate when she wrote that her 7-year-old son had come out to her, though she acknowledged he may not grow up to be gay.