Keira has decided to tell the Mail what happened to her, in order to highlight her plight and, she says, serve as a warning to others. Whatever the rights and wrongs of the protagonists in the legal action, her moving story gives us a piercing insight into the emotive area of transgender treatment

At first sight Keira Bell looks and sounds like a teenage boy.

She has a deep voice, a flat chest, and shaves most mornings to stop dark stubble covering her face.

It is only her tiny hands and delicate arms which tell you that she was born a girl.

They are the parts of her 23-year-old body that have not changed, despite the drugs prescribed to her after she visited Britain's most respected child gender-identity clinic, the Tavistock in London.

'I am constantly taken for a boy,' she says today. 'I get called 'Sir' when I speak to officials. I worry what women think when they see me using their loos or changing rooms. I am living in a world where I do not fit in as a male or as a female. I am stuck between the two sexes.'

For this tragic state of affairs Keira blames the treatment that began at the Tavistock in North London when, at 16, she no longer wanted to be a girl and asked for help.

After three one-hour appointments, she was prescribed hormone blockers to halt the development of her female body.

Put on what she calls a 'roller coaster' journey, she was soon being given the male hormone testosterone to change her appearance. Three years ago, she had her breasts removed, in an operation paid for by the NHS.

Despite that dramatic step, this story has taken an extraordinary twist. For Keira has now changed her mind about being a man. She believes it is an impossible quest, and is trying to reverse the process.

This week it was revealed she is the key witness in a landmark High Court case against the Tavistock and Portman NHS Foundation Trust, which runs the clinic.

The legal action questions the basis on which the Tavistock's Gender Identity Service obtains consent to treat youngsters — some of them as young as 12.

So controversial is the high-profile case that some lawyers expect it to end up in the Supreme Court, the UK's highest judicial body, for a decision on how gender reassignment treatment should be authorised for those who haven't reached adulthood.

This week, in a statement, Keira said: 'I don't believe children and young people can consent to the use of powerful, experimental hormone drugs like I did. Hormone-changing drugs and surgery do not work for everyone, and it certainly should not be offered to someone under 18.'

Keira (pictured as a little girl) blames the treatment that began at the Tavistock in North London when, at 16, she no longer wanted to be a girl and asked for help. After three one-hour appointments, she was prescribed hormone blockers to halt the development of her female body

Keira has decided to tell the Mail what happened to her, in order to highlight her plight and, she says, serve as a warning to others.

Whatever the rights and wrongs of the protagonists in the legal action, her moving story gives us a piercing insight into the emotive area of transgender treatment.

Keira was brought up in Hertfordshire, with two younger sisters, by her single mother, as her parents had divorced. Her father, who served in the U.S. military in Britain and has since settled here, lived a few miles away, and she saw him often as she was growing up.

She was always a tomboy. She did not like wearing skirts, and can still vividly remember two occasions when she was forced by her family to go out in a dress.

'Once was when I was five or six and a bridesmaid. I was nervous, but I did it,' she says.

'The other time I was in Tenerife with my mother's relatives. They expected me to wear a dress, so I did.'

She twists those elegant hands as she explains what happened next.

'At 14, I was pitched a question by my mother, about me being such a tomboy. She asked me if I was a lesbian, so I said no. She asked me if I wanted to be a boy and I said no, too.'

She changed her name and sex on her driving licence and birth certificate, calling herself Quincy (after musician Quincy Jones) as she liked the sound of it. She also altered her name by deed poll, and got a government-authorised Gender Recognition Certificate making her officially male. Yet her worries were growing. Keira is pictured above as a man

But the question set Keira thinking that she might be what was then called transsexual, and today is known as transgender.

'The idea was disgusting to me,' she tells me. 'Wanting to change sex was not glorified as it is now. It was still relatively unknown. Yet the idea stuck in my mind and it didn't go away.'

Keira's road to the invasive treatment she blames for blighting her life, began after she started to persistently play truant at school.

An odd one out, she insisted on wearing trousers — most female pupils there chose skirts — and rarely had friends of either sex.

When she continually refused to turn up at class as a result of bullying, she was referred to a therapist.

She told him of her thoughts that she wanted to be a boy. 'I felt I was not being listened to at school and blamed it on being a girl,' she explains.

'I did not feel respected as a young woman compared with young men. I thought life would be better for me if I changed my sex.'

Very soon, she was referred to her local doctor who, in turn, sent her to the child and adolescent mental health service (CAMHS) near her home. From there, because of her belief that she was born in the wrong body, she was given treatment at the Tavistock

It was a day she remembers well. 'My dad drove me to London, although he was very worried about the whole thing. I was, I admit, desperate to start my transition to male from female. I thought it would change everything.'

At the Tavistock, she says, there was 'no resistance' to her dream, even though she was little more than a child. Keira had entered puberty and her periods had begun. 'The Tavistock gave me hormone blockers to stop my female development. It was like turning off a tap,' she says.

'I had symptoms similar to the menopause when a woman's hormones drop. I had hot flushes, I found it difficult to sleep, my sex drive disappeared. I was given calcium tablets because my bones weakened.'

It seems incredible, but Keira claims she was not warned by the Tavistock therapists of the dreadful symptoms ahead. 'My female hormones had been flushing through my body and, suddenly, a curtain came down on them. It felt pretty bad,' she recalls ruefully.

Worse for her was the disappointment that her body did not suddenly change from female to male.

Her breasts, which she had been binding with a cloth she bought from a transgender internet site, did not instantly disappear. 'I was in nowhere land,' she says.

Yet back she went to the Tavistock, where tests were run to see if she was ready for the next stage of her treatment after nearly a year on blockers.

'I was prescribed regular injections of testosterone to make me physically change sex,' she says.

A few months later, she noticed the first wispy hairs growing on her chin. At last something was happening. Keira was pleased.

She was referred to the Gender Identity Clinic in West London, which treats adults planning to change sex.

After getting two 'opinions' from experts there, she was sent to a hospital in Brighton, East Sussex, for a double mastectomy.

'I was 20 and excited,' she says. 'I had been binding my breasts for years. I did not like their appearance. I wanted to get rid of them.'

By now, she had a full beard, her sex drive returned, and her voice was deep. 'My dad had given up on me.

He took me to Brighton but was upset. I went in one night and was operated on the next day by a top consultant. My breasts were gone.

'No one at the clinic sat me down beforehand and said: 'Are you certain you want this?' It was all very quick.'

Keira now believes she wasn't thinking straight, saying: 'My main reason for wanting the operation was that it meant I didn't have to flatten my breasts with the binder, which I'd used for years and is painful.'

It was at this stage that she began to have doubts about becoming a boy. Although she'd had teenage romances with both female and male partners, now she was lonely and felt she didn't fit in either world.

She started to work, first as an apprentice at a video gaming business, then at an electronics shop.

At the Tavistock, she says, there was 'no resistance' to her dream, even though she was little more than a child. Keira had entered puberty and her periods had begun. 'The Tavistock gave me hormone blockers to stop my female development. It was like turning off a tap,' she says

'The shop accepted me as a man. I used the male loos,' she says.

'But when I went out socially, which was rare, I never knew which loo to use. I was neither one thing nor the other.'

Despite her doubts, she pressed on. She changed her name and sex on her driving licence and birth certificate, calling herself Quincy (after musician Quincy Jones) as she liked the sound of it. She also altered her name by deed poll, and got a government-authorised Gender Recognition Certificate making her officially male.

Yet her worries were growing. 'I didn't want to tell my dad I had made a mistake,' she recalls. 'It was embarrassing changing my mind. When I went online to social chat rooms to talk about it, the pro-transgender lobby kept saying: 'Oh, it's normal to have doubts.' '

Finally, she took action. In January last year, soon after her 22nd birthday, she had her final testosterone injection. They were always given every few months by a nurse at her GP surgery. 'I decided never to go again,' she says.

But, after years of having hormones pumped into your body, the clock is not easily turned back. It is true that her periods returned and she slowly began to regain a more feminine figure around her hips. Yet her beard still grows.

'I don't know if I will ever really look like a woman again,' she admits. 'I feel I was a guinea pig at the Tavistock, and I don't think anyone knows what will happen to my body in the future.' Even the question of whether she will be able to have children is in doubt.

She has started buying women's clothes and using female loos again, but says: 'I worry about it every time in case women think I am a man. I get nervous. I have short hair but I am growing it and, perhaps, that will make a difference.'

Last year she plucked up the courage to tell her father about her change of heart.

The man who cried when his daughter declared she was becoming a boy, cried again.

'He was so happy. I think he hopes I will suddenly become very girly and buy dresses,' she says.

'He gave me some bath gels for a Christmas present. They were very feminine and prettily wrapped. I haven't used them yet, but I probably will,' she says with a smile.

There is another problem. By law she is male, and she faces the bureaucratic nightmare of changing official paperwork back to say she is female.

This week it was revealed she is the key witness in a landmark High Court case against the Tavistock and Portman NHS Foundation Trust, which runs the clinic. The legal action questions the basis on which the Tavistock's Gender Identity Service obtains consent to treat youngsters — some of them as young as 12. She is pictured above with Susan Evans, who is leading the action

'If I committed a crime, I would be put in a male prison,' she explains. 'I want to get the gender recognition document annulled.'

Campaigners supporting Keira's court battle say the number of young people regretting a sex change is rising.

A new charity, The Detransition Advocacy Network, has been set up to help them. Its founder Charlie Evans was born female but lived as a man for nearly a decade before accepting her birth sex.

She says she has been contacted by 'hundreds of young adults' — some only 19 or 20 — who claim the treatment has not solved their problems.

Yet, demand for gender identity treatment is growing. More than 13,500 people are on waiting lists, according to BBC research earlier this month.

The Government has also launched an inquiry into the explosion in the number of children wanting to change sex.

In 2009/10, 40 girls under 18 were referred to doctors for gender treatment in England.

By 2017/18, the number had soared to 1,806. Over the same period, annual referrals for boys increased from 57 to 713.

The Tavistock and Portman NHS Foundation Trust has told the Mail that their gender identity service has an international reputation for being cautious about treatments and is monitored closely by NHS England.

'It has a high level of reported satisfaction and is rated 'Good' by the Care Quality Commission,' said a spokesperson. The Trust has refused to comment further because of the ongoing legal action.

However, in the High Court this week Keira's barrister, Jeremy Hyam QC, explained: 'What is challenged is the clinic's current and continuing practice of prescribing puberty-suppressing hormone blockers and, subsequently, cross-sex hormones, to children under the age of 18.'

Keira herself said: 'The treatment needs to change so that it does not put young people, like me, on a torturous and unnecessary path that is life-changing. I feel like I've been lied to because it did not make me feel any better.'

As she struggles to return to life as a woman, she adds, with feeling: 'I don't want any more kids to suffer like me.'

crowdjustice.com/case/protect-children/