A record 50 children a week are being referred to gender realignment doctors.
Kids aged just four are among those believing they want to change their sex.
One specialist said: “This is a massive increase.”
The growing acceptance of gender issues may have fuelled the rise in the number of children being referred, experts believe.
Fifty a week are being sent to see specialists because they are unhappy with the body they were born in, figures obtained by the Mirror show.
Those aged 11 or over may be treated with powerful hormones to prevent them going through puberty.
Once they reach a more mature age they can decide whether they still want to go ahead with gender realignment.
If they do, girls are given testosterone and boys oestrogen to trigger the change.
Gender dysphoria expert Professor Ashley Grossman insists this puberty blocking treatment “does not harm children” but gives them extra time to make the crucial decision.
The Oxford University endocrinologist said: “Many of these children have very mild gender dysphoria and are not very happy in their gender but puberty is a very confusing time. Others are clearly trapped in the wrong body.
“We have used these treatments for children who, for whatever reason, start puberty early, some as young as seven which can be very distressing.
“These drugs just buy them another couple of years before they decide to do anything permanent.
“They can then get to the age of 16 or 17 and say, ‘Things are starting to sort themselves out, I want to go through puberty.’ Others do not.
“There is a bit of a call from the public that they are being over-diagnosed and people get disturbed about medicating children but with puberty blockers they are not being treated, just given time.
“There are no legal restrictions but I would feel very uncomfortable about giving a child hormones to change their gender or sex before they are mature.”
The number of children visiting Britain’s specialist clinic the Gender Identity Development Service, hosted by the Tavistock and Portman NHS Foundation Trust in London, has risen by 24% to 1,302 in the past six months.
Among them were two children aged four, four five-year-olds and 17 kids of six. In 2016/17 there were 2,016 children referred to the clinic. This is on track to rise in 2017/18 to 2,600. In 2009 there were 97. Prof Grossman said: “This is a massive increase. We don’t know whether this is a true increase or that these kids in the past just didn’t know this service existed.
“I only deal with adult patients but the majority tell me they realised they had gender problems when they were young but hadn’t seen a medical professional until an adult. There is more realisation now.”
GIDS director and clinical psychologist Dr Polly Carmichael added: “There is no single explanation for the increase.
“But we do know there has been significant progress towards the acceptance and recognition of transgender and gender diverse people in our society, coupled with greater knowledge about specialist gender clinics and support available.”
Around 40% of children who attend GIDS are prescribed puberty blocking drugs. But gender expert Professor Miroslav Djordjevic suggested the rise could be in part a fad among parents who indulge their children.
He said: “I cannot believe the 50 a week will all be transgender. We’re seeing a lot of parents who come for help because they are not sure if there children are transgender or just being children,
“To say a girl of five is transgender is impossible. But it is easier to ask than not do anything and children suffer. At the minute it’s something like a fashion.
“And of course there is a danger children could regret having treatment.”
Mirror agony aunt Dr Miriam Stoppard believes the rise reflects “parental concern” about their children’s wishes.
She added: “We have to understand the importance of not consigning someone to a gender they feel they aren’t, by letting them go through puberty. That’s inhuman and inhumane. The first prerequisite for a child with gender dysphoria is to have parents on their side.
“This is an important driver of the increased numbers. No parent can bear their child to be stuck in a gender no man’s land.
“I’m sure the universal acceptance of LGBTQ pioneers has helped legitimise and publicise gender dysphoria and its treatment.”
Singer Paloma Faith has told how she is bringing her first child up as gender neutral. The 36-year-old refuses to reveal the sex of her baby, born in December. She said: “I want three kids and they’ll be gender neutral.”
My child, 8, goes to school as a girl. It is not 'just a phase'
A mum whose eight-year-old child started school as a girl has called for more investment in gender realignment clinics.
Tegan Dyason, born as Tom, has been wearing girls’ clothes since the age of three and began visiting gender specialists last year. Her mum Michelle has called for people to accept it is “not just a phase”.
She has welcomed news more children have been referred for treatment and called for even more to be spent.
“I have spoken to a lot of parents but the majority are on waiting lists to get in to clinics,” she said. “We need more investment in these medical services.
“We were lucky, we got seen quickly, but there’s more children and teenagers experiencing gender issues and, more than anything for the children’s safety, money is needed to make sure they are being dealt with as intensely as they need to be.
“I would hate to see them starting to rush people through. I think that’s when mistakes could be made.”
Tegan will have years of assessment before she is old enough to be considered for medical intervention at puberty.
'I regret treatment... giving it to kids could be perpetuating a great harm'
A woman who began treatment to become a man and later regretted it has warned against letting young children have such procedures.
Kate, 30, who spoke on condition of anonymity, injected herself with hormones, causing her voice to drop and facial hair to grow. She gave up after deciding she was not transgender.
The medical student, who started her treatment in her early 20s, said she felt encouraged to do so after visiting online forums. She said: “I’m very concerned that if I was a teenager now or even younger, I or my parents would be pushed to consider me then as transgender. I would have welcomed that at the time.
“I wanted to be a boy when I was younger because boys were allowed to be assertive and confident.
“A young person may now take hormones or have surgery and later regret it. By giving treatment to children we may be perpetrating a great harm.”
Kate said only last year did she become comfortable as a female. She said her deeper voice and facial hair are a constant reminder of how “misguided” she was.
Four careful stages that lead to body's final change
The Gender Identity Development Service is seen as a world leading centre for treatment of dysphoria.
It adopts a “staged model of care” to guard against children taking steps to change their sex and then regretting it.
Kids as young as three are offered counselling along with their parents when they first present at the clinic.
This can last for years. Assessments are done around self-harm and suicide risk. Just before puberty the patient can be prescribed hormone blockers.
The full psychological effects of these drugs, or whether they alter the development of the brain, is unknown.
But they allow the youngster time to explore their developing gender identity with a team of specialists.
The third stage, from age 16, is to prescribe either oestrogen or testosterone after a minimum of 12 months on hormone blockers.
This can cause irreversible changes to the body such as growth of breast tissues. Doctors require the person to be in education or employment in their preferred gender role.
Stage four is surgery to change the sexual organs.