Five-year-old Zachery Jack Avery’s wardrobe contains a purple tutu, a silky mauve blouse and a floral patterned skirt.

Zachery Jack Avery is one of the youngest known children to be backed by the NHS in UK after being diagnosed with Gender Identity Disorder (GID).

The boy has a long fair hair done up in bunches as he has been insisting on living as a member of the opposite sex.

The extraordinary conclusion that he is a girl trapped inside a boy’s body is being backed by medical experts and supported by his school, which has made a lavatory block “gender neutral” to accommodate his needs.

Until shortly before his fourth birthday in December 2010, Zachy, as his mother Theresa calls him, was “a normal little boy”. He loved to play with his Thomas the Tank Engine train set alongside his brother Alex.

Suddenly, however, Zachery Avery became obsessed with the children’s TV cartoon character Dora the Explorer, and started to dress in girls’ clothes.

Theresa Avery, who has two other children, said: “He just turned round to me one day and said, <<Mummy, I’m a girl>>. I assumed he was just going through a phase and 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.”

Theresa Avery, 32, and husband Darren, 41, became increasingly worried by their son’s behavior and took him to a doctor, then to a child psychologist.

At the Tavistock and Portman NHS Foundation Trust in London, which specializes in the disorder, Zachery Avery was diagnosed as having GID.

His mother said: “They told us that although he had a male body, his brain was telling him he was a girl.”

At home in Purfleet, Essex, a first-floor flat in a block on a 1980’s estate, Darren and Theresa Avery continued to allow Zach to dress in girls’ clothes and play with his sister’s dolls.

Meanwhile at the boy’s school, Purfleet Primary, they have applied themselves to what Theresa Avery calls “the bathroom issue” and are hoping Zach can use the staff toilets when he gets older.

About the other pupils, she said: “We explained to the other kids that Zach’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. They haven’t batted an eyelid. They’ve accepted Zach as Zach and there’s been no problems at the school with bullying. The school has been brilliant and really, really supportive.”

Purfleet Primary, which has 350 pupils up to age 11, was given the lowest “inadequate” rating in December after an Ofsted inspection. Teachers allow Zachery Avery to wear a girl’s trouser uniform and black boots with pink trim, which his mother says is female but still neutral.

Theresa Avery admits she misses her little boy, but she adds: “He just wants to be like a little girl. He’s very happy with his long 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 Doctor Who 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.”

The mother added: “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.”

On his Facebook page, Darren Avery describes himself as an artist and “dad of four starting my own business drawing pictures from photos’. Sketches he has uploaded include a reclining nude woman wearing a necklace with a large, heart-shaped pendant, based on Kate Winslet in Titanic. Others include a blonde girl striking a provocative pose in a figure-hugging top, and a faceless female wearing only a Mickey Mouse T-shirt.

Yesterday Darren and Theresa Avery were negotiating with a tabloid newspaper to sell Zachery’s story for a five-figure sum.

Figures from the Tavistock and Patman clinic – the national body for GID – revealed that 165 children have been diagnosed with the condition this year.

A spokesman said they were unable to comment on individual cases, but only seven children under five were diagnosed last year – making Zach one of the youngest.

“Tavistock Clinic had 97 referrals in 2009/2010; 139 in 2010/2011 and 165 thus far this year.

“The trend in referrals has been up over the years – this may reflect greater awareness.

“We see children and young people up to the age of 18, from across the UK, who are experiencing difficulties in the development of their gender identity.

“This includes children who are unhappy with their biological sex. Some may be boys who prefer activities and roles associated with the opposite sex, some may also identify as the opposite sex and vice versa for girls. In general when younger children are referred it is in relation to cross-gender preferences in play, playmates and activities. It is more unusual for children of this age to express cross-gender identification – that is the wish or belief that they belong to the opposite sex.

“The diagnosis of GID is made by the key workers working with the young person. We will also assess their general well-being. 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.”