NHS Tavistock Clinic treating transgender children with therapies to prepare them for sex change Transgender children as young as 12 are being given hormone blockers They are being prescribed by doctors at Tavistock Clinic in north London Hormone blockers halt the onset of puberty and sexual characteristics Treatment is generally denied to children under the age of 16 An NHS clinic is treating transgender children as young as 12 with drug therapies to prepare them for a sex change, it has been revealed. More than 20 youngsters with 'gender dysphoria' are being prescribed hormone blockers by doctors at the Tavistock Clinic in north London. The treatment halts the onset of puberty - preventing children from developing the sexual characteristics of the gender they were born. Controversial: The Tavistock Clinic in north London is prescribing hormone blockers to children as young as 12 The clinic started offering the medication in 2011 as part of a study to determine whether transgender children could benefit from starting treatment earlier than in the past, according to The Sunday Times. It was initially deemed controversial due to the patients' ages - with critics arguing that the youngsters lacked the ability to consent to the therapies. However, since then, the clinic has received a staggering 142 referrals of children aged 11 to 15 from parents and carers. Dr Polly Carmichael, director of the Tavistock Clinic's gender identity service, said she plans to continue offering the treatment to youngsters after the study culminates in April. Fight: Transgender Leo Waddell, 12, was born as a girl called Lily, right. Last month, he vowed to fight a doctor's decision to ban him from taking hormone blockers, which halt the onset of puberty 'Thirty-five [children] so far have been accepted into the early intervention study, about half of these born as girls and half as boys, of whom 23 are already being treated with hormone blockers,' she told The Sunday Times. Another 12 have gone forward but are not yet in treatment, perhaps because they are still too early in puberty. Once they have reached 16 years old, children are given two options: To halt the hormone blockers - causing them to stay the same gender - or to take 'cross-sex' hormones, which change their body to the opposite gender. At the age of 18, the Tavistock Clinic is able to refer them for a sex change operation. Upset: Speaking on This Morning last month, Leo - pictured with his mother, Hayley, 48 - said he started to cry when he was told he couldn't undergo hormone therapy The clinic's approach is a sharp contrast to most doctors' surgeries in the UK, where children under 16 years old are generally refused hormone blockers. However, pro-treatment campaigners argue that, by their 16th birthday, most youngsters have experienced the onset of puberty - making it harder for them to change gender.. Dr Carmichael said it was 'better' for children not to have gone through puberty before 'transitioning'. But she added: 'You are asking someone aged as young as 11 to make big decisions about their adult life and identity. We have to be very careful to keep options open. Beauty Queen: Miss England finalist Jackie Green became the youngest person in the world to undergo transgender surgery Last month, 12-year-old transgender Leo Waddell vowed to fight a doctor's decision to ban him from taking hormone blockers. Leo, from Lowestoft, Suffolk, was born as a girl called Lily but has lived as a boy since the age of five. He plans to take testosterone at the age of 16, before undergoing gender reassignment surgery at the age of 18. In the meantime, he wants to take hormone blockers to prevent the onset of puberty and the release of female hormones in his body. However, despite undergoing numerous psychological and hormone tests, his GP has refused to prescribe them because she was unsure of the long-term effects - adding: 'Any clinical decision I make always gives consideration to every aspect of the patient's wellbeing'. Speaking on This Morning with his mother, Hayley, 48, Leo said: 'I didn't like it, I started to cry when she told me.' Mrs Waddell added: 'What so many people don't understand is that I feel it's far more dangerous for Leo to not have these hormone blockers. The things he has to go though, the emotions and the torment is slowly bringing him down and down. Leo is one of a growing number of British children with gender dysphoria, a condition whereby you feel have been physically assigned the wrong gender at birth. In 2009, aged 16, Jackie Green became the youngest person in the world to undergo transgender surgery - having travelled to America aged 12 to receive hormone blockers. Last year, as a 19-year-old woman, Jackie made history once again by becoming the first transgender Miss England finalist. She later claimed she 'would have killed' herself if she hadn't been prescribed the blockers by a doctor in Boston.