Papers have been lodged at the High Court by a mother and a nurse against the Tavistock and Portman NHS Trust, which runs the UK’s only gender-identity development service (Gids).
Lawyers will argue it is illegal to prescribe the drugs, as children cannot give informed consent to the treatment.
The Tavistock said it had a “cautious and considered” approach to treatment.
Papers were formally lodged on Wednesday by the lawyers for the complainants.
The Tavistock will have three weeks to submit its response, after which a judge will consider the case and decide on the timings of any hearings.