Three senior judges have ruled that children under 16 are unlikely to be able to give informed consent to treatment involving puberty-blocking drugs.
Puberty blockers are prescribed to some children who are experiencing gender dysphoria, to temporarily stop their bodies developing.
The NHS describes gender dysphoria as "a sense of unease that a person may have because of a mismatch between their biological sex and their gender identity" which in some cases may be "so intense it can lead to depression and anxiety and have a harmful impact on daily life".
The drugs suppress the release of hormones produced in much bigger quantities during puberty.
The hormones are messengers telling your body to develop things like breasts, periods, facial hair or a deeper voice.
The blockers are also used to treat conditions which cause premature puberty in much younger children.
Legal action was launched against the NHS in January over the prescribing of the drugs.
Why are they used?
The Gender Identity Development Service (Gids), based in London and Leeds, says that pausing puberty is designed to give a young person with gender dysphoria more time to consider their options - while not having to go through the additional distress of their body changing in a way they do not want.
When someone stops taking blockers, their puberty should resume.
By pausing puberty and the development of things like breasts or facial hair, someone who goes on to have cross-sex hormone therapy may avoid having more invasive surgical treatment like having their breasts removed (mastectomy) later on.
Cross-sex hormone therapy involves taking either oestrogen or testosterone and is a treatment only available to over-16s on the NHS in England.
The NHS trusts involved argue taking puberty blockers and cross-sex hormones are completely separate stages of treatment, though the limited evidence that exists suggests that most young people beginning the first treatment go on to the second.
Why have they been controversial?
The legal action against the Tavistock and Portman NHS Trust, which runs the gender identity clinic, focused on whether children could give informed consent to treatment with puberty blockers.
Puberty hormones are linked to changes not just in the body but also in the brain.
Gids says that it is not yet known whether puberty blocker treatment "alters the course of adolescent brain development".
It also says that the full psychological effects of the blocker are not yet known.
Some early data from one study showed some taking the drugs reported an increase in thoughts of suicide and self-harm, but it was unable to say whether it was the drugs or something else causing the increase.
Experts on clinical trials criticised the design of the study but said the data warranted further investigation.
NHS England says: "Little is known about the long-term side effects of hormone or puberty blockers in children with gender dysphoria.
"Although the Gender Identity Development Service advises this is a physically reversible treatment if stopped, it is not known what the psychological effects may be.
"It's also not known whether hormone blockers affect the development of the teenage brain or children's bones. Side effects may also include hot flushes, fatigue and mood alterations".
It has also been claimed puberty blockers may effect someone's fertility and sexual functioning, although the evidence around this is limited.
One of the people bringing the court case, known as Mrs A, is the mother of a 15-year-old with autism who is on the waiting list for treatment at the Gids.
Gids says that there seems to be a "higher prevalence of autistic spectrum conditions in clinically referred, gender dysphoric adolescents than in the general adolescent population".
The judges said their decision was only on the informed consent of a child or a young person, not whether puberty blockers were appropriate themselves.
Increase in referrals
There has been a large increase in children being referred to Gids in recent years.
While there is no clear explanation for this increase in referrals, Gids puts it down to greater awareness.
Large increases in referrals have also been seen at gender identity clinics elsewhere, for example in the US and Canada.
There has also been a shift in the past few years, with people assigned female at birth making up the bulk of the increase in young people wanting to transition.
Who can they be prescribed to?
Puberty blockers, when used to treat gender dysphoria, can only be initially prescribed by a specialist, rather than a GP.
They are prescribed by the NHS to children or young people who have already started puberty. This process begins often years before any physical signs of puberty appear and has to be assessed by a hormone specialist.