Trans children should not receive controversial puberty blockers unless they understand the risks, according to a landmark High Court ruling, as judges warn that most teenagers cannot give their consent. The ruling means that children who wish to undergo gender reassignment can now only legally consent to taking puberty blockers if they are able to understand the “long-term risks and consequences of the administration of” the drugs. The case had been brought against Tavistock and Portman NHS Trust, which runs the UK’s only gender identity development service (GIDS) for children, by Keira Bell, a 23-year-old woman who began taking puberty blockers when she was 16 before “detransitioning”. She said the clinic should have challenged her more over her decision to transition to a male as a teenager. The legal challenge was also brought by a woman who can only legally be identified as ‘Mrs A’, the mother of a 15-year-old autistic girl who is currently on the waiting list for treatment. At a hearing in October, their lawyers said children going through puberty are “not capable of properly understanding the nature and effects of hormone blockers”. They argued that there is “a very high likelihood” that children who start taking hormone blockers will later begin taking cross-sex hormones, which they say cause “irreversible changes” and that the NHS Trust offers "fairytale" promises to children because they are unable to give their consent to the sex-change process. However in the judgement handed down on Tuesday, Dame Victoria Sharp, sitting with Lord Justice Lewis and Mrs Justice Lieven, said that children under 16 needed to understand “the immediate and long-term consequences of the treatment” to be able to consent to the use of puberty blockers. The judges said in their ruling: “It is highly unlikely that a child aged 13 or under would be competent to give consent to the administration of puberty blockers. “It is doubtful that a child aged 14 or 15 could understand and weigh the long-term risks and consequences of the administration of puberty blockers.” They added: “In respect of young persons aged 16 and over, the legal position is that there is a presumption that they have the ability to consent to medical treatment. “Given the long-term consequences of the clinical interventions at issue in this case, and given that the treatment is as yet innovative and experimental, we recognise that clinicians may well regard these as cases where the authorisation of the court should be sought prior to commencing the clinical treatment.” During the High Court hearing in October, the Tavistock and Portman NHS Trust - as well as University College London Hospitals NHS Foundation Trust and Leeds Teaching Hospital NHS Trust, to which Tavistock refers children and young people experiencing gender dysphoria - argued that taking puberty blockers and later cross-sex hormones were entirely separate stages of treatment. But, in its ruling, the High Court said: "It is said therefore the child needs only to understand the implications of taking puberty blockers alone ... in our view this does not reflect the reality. "The evidence shows that the vast majority of children who take puberty blockers move on to take cross-sex hormones." The court added that both treatments were "two stages of one clinical pathway and once on that pathway it is extremely rare for a child to get off it". Speaking outside the Royal Courts of Justice after the ruling, Keira Bell said she was "delighted" with the High Court's ruling, adding that “common sense has prevailed”. "This judgment is not political,” she said, “it's about protecting vulnerable children." A statement was also read on behalf of her fellow claimant, Mrs A, which said: "I'm relieved to hear the court have understood and agreed with our concerns about... treating children and young people with puberty blockers." Their solicitor Paul Conrathe said the ruling was "an historic judgment that protects children who suffer from gender dysphoria". He added: "Ultimately this case was decided on the facts that were known by the Tavistock. "Ironically - and as matter of serious concern - despite its international reputation for mental health work, this judgment powerfully shows that a culture of unreality has become embedded in the Tavistock. "This may have led to hundreds of children receiving this experimental treatment without their properly informed consent." In response, a spokesperson for the Tavistock and Portman NHS Foundation Trust said: “The Trust is disappointed by today’s judgment and we understand that the outcome is likely to cause anxiety for patients and their families. “Our first duty is to our patients, particularly those currently receiving hormone blocking treatment and we are working with our partners, University College London Hospitals NHS Foundation Trust and Leeds Teaching Hospitals NHS Trust, to provide support for patients concerned about the impact on their care. “The Trust is seeking permission to appeal the judgment, and in the meantime, confirms its ongoing support for the review commissioned by NHS England being led by Dr Hilary Cass. “We will update our statement once we know the outcome of today’s further court proceedings.” Lui Asquith, from the trans children's charity Mermaids, said the ruling was a "devastating blow for trans young people across the country”. “We believe very strongly that every young person has the right to make their own decisions about their body and that should not differ because somebody is trans. "The court today has decided to treat trans young people differently to every other child in the country. "We believe that we're entering a new era of discrimination, frankly. We see day in, day out at Mermaids the positive impact hormone blockers can have on some trans young people - in all honesty, they can save lives. "They allow some young people to be able to go outside, engage in society, go to school, and we're now in a position whereby those young people are not necessarily going to be able to access it. "We're entering a new era of experimentation, that experiment being what happens to trans young people who need hormone blockers who can't get them."