The number of children aged 10 or under who have been referred to the NHS to help deal with transgender feelings has more than quadrupled in the last six years, according to new figures.
The Tavistock and Portman NHS Trust, the UK's only centre specialising in gender issues in under 18s, said the number of under 11s referred to the unit had risen from 19 in 2009-10 to 77 in 2014-15.
Over that period, 47 children were aged five or younger and two of the children were three years old.
The trust, which has clinics in London and Leeds, said gender dysphoria in young people is a "complex and rare condition" which is "frequently associated with distress which may increase at puberty".
A spokesman said: "It is probably fair to say that young people are increasingly interested in exploring gender.
"There is not one straightforward explanation for the increase in referrals, but it's important to note that gender expression is diversifying, which makes it all the more important that young people have the opportunity to explore and develop their own path with the support of specialist services."
Two children, Lily, six, and eight-year-old Jessica - which are not their real names - were born boys but became unhappy with their gender from an early age, according to their parents.
Speaking to the BBC's Victoria Derbyshire programme, Lily said: "If I had to live as a boy I would be really upset. But now I'm sort of living as a girl I feel much better."
Jessica added: "I really didn't want to be a boy. It was really frustrating for me. It feels like I'm in the wrong body."
Jessica's mother Ella - also not her real name - said a relative accused her of "conditioning" her son and an anonymous call was made to the NSPCC claiming Jessica's parents were "forcing their boy to live as a girl".
She told the programme: "There is nothing we have done to make this happen. You couldn't put a little boy in a dress if he didn't want to wear it."
The charity Mermaids, which supports parents with children who are uncomfortable with their gender, said it had been contacted by 60 families in the last three months.
Its chairwoman Susie Green said: "More parents are seeking help now than trying to make it go away or ignore it. There is greater awareness now and more information in the media and more sensitive reporting in the press.
"We help children who are seriously distressed. We don't know what it's going to be like for them in two years but it's important to help them in the here and now."
The Tavistock and Portman NHS Trust said that while young children may fulfil the criteria for a diagnosis of gender dysphoria, it "would not generally consider it helpful to make a formal diagnosis in very young children".
Children are offered counselling and support sessions and any physical intervention is not considered until a child approaches puberty, when hormone blockers might be offered, the trust said.
Blockers delay the physical change of puberty, allowing a young person time to further explore their gender identity and live as a man or woman in the longer-term, after which a patient can consider taking cross-sex hormones at the age of 16, and surgery after 18, it added.