An NHS clinic is treating children as young as 12 to prepare them for a sex change.
More than 20 young people 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.
The clinic started offering the medication in 2011 and has since received 142 referrals of children aged 11 to 15 from parents and carers.
Dr Polly Carmichael, director of the Tavistock Clinic's gender identity service, told the Sunday Times: "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.
"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 start taking 'cross-sex' hormones and begin the process of changing sex.
At the age of 18, the Tavistock Clinic is able to refer them for a sex change operation.
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."