The High Court is hearing a test case this week into whether children and teenagers can give informed consent to gender reassignment treatment.
Opponents say that children under the age of 18 should not be allowed to use puberty blockers or cross-sex hormones because they are not able to give valid consent.
The judicial review has been brought by Susan Evans, a former staff member at the Tavistock and Portman NHS Trust, the only gender identity development service (Gids) on the NHS in the UK.
She has raised over £20,000 through crowdfunding to cover the costs of the legal challenge against the trust and NHS England.
She argues that "largely experimental" medical treatment for gender reassignment will put under-18s at unknown risk.
"The harm they might suffer could have lifelong consequences. Many professionals are now highly concerned about the treatments for under-18 gender dysphoric children and adolescents which remain largely experimental," she said, according to The Times.
"There are so many unanswered questions that include the age at start, reversibility, serious adverse health events, long-term effects on mental health, neurological effects on cognitive functioning, the effect on bone density, circulatory systems and sexual functioning in adulthood.
"We cannot stand by and watch young people be part of an experimental medical treatment that exposes them to very significant risks."
She and "Mrs A", the mother of an autistic 15-year-old girl on the Gids waiting list, are being represented by Paul Conrathe, a solicitor with Sinclairslaw, who told The Observer: "We are essentially seeking to say that the provision at the Tavistock for young people up to the age of 18 is illegal because there isn't valid consent."
A spokeswoman for the Tavistock and Portman NHS Trust told The Times that its "clinical interventions are laid out in nationally set service specifications".