Media playback is unsupported on your device Media caption Keira Bell: 'I should have been challenged on my transition'

A 23-year-old woman who is taking legal action against an NHS gender clinic says she should have been challenged more by medical staff over her decision to transition to a male as a teenager.

A judge gave the go-ahead for a full hearing of the case against the Tavistock and Portman NHS Trust.

Lawyers will argue children cannot give informed consent to treatment delaying puberty or helping them to transition.

The Tavistock said it always took a cautious approach to treatment.

Gender identity charity Mermaids said that people face a long wait for access to such services, that they can save lives and that very few people regret their decision.

The clinic based in Hampstead, north-west London, which runs the UK's only gender-identity development service (GIDS), added that it welcomed an examination of the evidence in this contentious area.

Keira Bell is one of the claimants and will give evidence in the judicial review, which is likely to be heard in early summer.

The second claimant, known only as Mum A, is the mother of a 15-year-old girl with autism, who is awaiting treatment at the clinic.

Keira describes being a tomboy as a child. When asked how strongly she felt the need to change her gender identity, she replied that it gradually built up as she found out more about transitioning online.

Then as she went down the medical route, she said "one step led to another".

She was referred to the Tavistock GIDS clinic at the age of 16. She said after three one-hour-long appointments she was prescribed puberty blockers, which delay the development of signs of puberty, like periods or facial hair.

She felt there wasn't enough investigation or therapy before she reached that stage.

"I should have been challenged on the proposals or the claims that I was making for myself," she said. "And I think that would have made a big difference as well. If I was just challenged on the things I was saying."

What are puberty blockers?

They are drugs which can pause the development of things like breasts, periods, facial hair and voice breaking

They can be prescribed to children with gender dysphoria who feel their sex at birth doesn't match up with their gender.

This is meant to give them more time to weigh up their options before they go through the physical changes of puberty.

Although puberty blockers are described by the NHS as reversible, GIDS acknowledges that their impact on brain development and psychological health is not fully known.

A year after starting the puberty-blockers she said she was prescribed the male hormone testosterone, which developed male characteristics like facial hair and a deep voice. Three years ago, she had an operation to remove her breasts.

"Initially I felt very relieved and happy about things, but I think as the years go on you start to feel less and less enthusiastic or even happy about things.

"You can continue and dig yourself deeper into this hole or you can choose to come out of it and have the weight lifted off your shoulders."

'Too young'

She decided to stop taking cross-sex hormones last year and said she was now accepting of her sex as a female. But she was also angry about what had happened to her in the last decade.

"I was allowed to run with this idea that I had, almost like a fantasy, as a teenager.... and it has affected me in the long run as an adult.

"I'm very young. I've only just stepped into adulthood and I have to deal with this kind of burden or radical difference - in comparison to others at least."

Keira's lawyers will argue that children cannot weigh up the impact such a treatment might have on their future life, including for instance, on their fertility.

Former staff at the clinic have raised concerns that teenagers who want to transition to a different gender are being given puberty blockers without adequate assessment or psychological work.

It has been claimed that children as young as 12 have received the drugs, which block the hormones that lead to puberty-related changes like periods or facial hair.

But she also understands why teenagers arrive at the clinic deeply distressed and desperate to change their gender.

"I did say the same thing years ago when I went to the clinic. I would say it was saving me from suicidal ideation and depression in general and at the time I felt it relieved all those mental health issues I was feeling, alongside gender dysphoria."

Media playback is unsupported on your device Media caption Gender transitioning 'saves lives', says charity chief

She described her family life as difficult. She also believes if she had felt more accepted by society as she was then, she might not have wanted to change her gender. She added that she wouldn't have wanted to listen to voices of caution when she was younger.

"I feel I could say anything to my 16-year-old self and I might not necessarily listen at that time. And that's the point of this case, when you are that young you don't really want to listen.

"So I think it's up to these institutions, like the Tavistock, to step in and make children reconsider what they are saying, because it is a life-altering path."

Dr Polly Carmichael is the consultant clinical psychologist who runs the Gender Identity Development Service. She praised Keira for speaking out, but insisted the clinic did have a thorough assessment process.

'Really complex'

She described their approach as cautious and said they work closely with children and their families to reach the right decisions for them, with fewer than half of those seen going onto take puberty blockers or cross-sex hormones.

"This is a really complex area with strong feelings on all sides. And at its centre, the young people we work with - they come to us in often really great distress around their sense of themselves.

"We're talking about identity here, their identity, and a feeling that their gender identity does not match that body."

She believes the judicial review, when it happens, will be an important opportunity to ensure the evidence around treatment and a child's ability to consent is thoroughly examined.

"This is a heated debate at the moment. And I think taking a step back - and having an external considered review of the evidence and people's feelings about the most appropriate way to support young people - can be nothing but beneficial at this point."

Waiting time

Gender identity charity Mermaids provides support to trans and gender-diverse young people and their families.

Its chief executive, Susie Green, has defended the current process, which she said was based on years of research, and said she hoped the judicial review would "shine a light" on young people's experiences.

She told BBC News that many people who approached the charity were "very distressed" and that research had suggested puberty blockers could help reduce rates of self-harm and suicide.

And she said it was "not proportionate" to take away services because of "a very small number" of people who regretted undergoing medical intervention.

"In the first instance the waiting time is well over two years and when young people get into the service there is then a process which takes well into a year before medical intervention is considered," she told BBC News.

"The process is very detailed they get a lot of information about the benefits, the pitfalls and the projected outcomes of what comes of any kind of medication. So they make informed consent and that underpins the NHS."

NHS England is an interested party in the legal case. It has already announced an independent review of its policies on the use of puberty blockers and cross-sex hormones.

It describes this as part of a planned examination, which will be undertaken by a panel of independent experts.