Young people with gender identity issues are suffering "psychological torture" because of how long it is taking to receive specialist NHS help.
Teenage referrals to the Tavistock Centre in London have risen 42% in a year, with 2,016 referrals in 2016-17.
This follows a rise of 104% from 697 referrals in 2014-15, to 1,419 the year after.
Expert Terry Reed said for those already approaching or were in puberty, the fear they experienced was extreme.
Some 90% of people referred to the centre are from outside the city, meaning Londoners are having to wait up to two years for an appointment.
The hospital said it was working hard to hire more clinicians.
Bernard Reed from the Gender Identity Research and Education Society (GIRES) said: "The wait is hugely distressing.
"Every day is an agony as they see themselves being more and more developing into a physical body which is quite foreign to them."
He added: "We should be very worried about the situation these young people are in. Research indicates that 84% have had suicidal thoughts, 40% may even have attempted suicide - that's a major, major risk. Waiting for treatment is damaging and dangerous."
He said more local services were needed outside London.
'It feels horrible, I was suicidal'
Alex, a 16-year-old from north London, was told when he was referred there would be a seven month waiting list, but that ended up being two years to receive the help he needed.
"I felt like I was having to put my whole life on hold to wait for this thing I needed, with no idea at the time because it was so unclear how long it would be and how long each assessment took.
"My physical transition felt so far away that I couldn't really see myself ever getting there."
He added: "I'm hoping to feel more comfortable in myself and get surgery and testosterone. It's so frustrating. It's not fair."
Helen, Alex's mother, described the process as "desperate".
"We were really very frightened and alone, we didn't know what to do.
"Nobody seemed to be able to advise us. Everyone was saying they didn't know anything about gender issues and it was The Tavistock who would advise on that, but we couldn't see the Tavistock for months and months."
Dr Polly Carmichael, director of the Gender Identity Development Service, said: "I really appreciate how frustrating and difficult it is, when you've finally perhaps plucked up enough courage to tell people about how you feel, then you have to wait, it's really hard. We're acutely aware of that."
She said the centre was working with NHS England to hire more clinicians and to offer groups whereby young people could talk to older people who had been through the service so there was a network of support while they waited for individual appointments.