The puberty blockers trial has already proved controversial, with campaigners threatening legal action.
Keira Bell, who took the Tavistock gender clinic to court in 2020 after she was given puberty blockers and cross-sex hormones as a teenager, said the trial should be halted immediately. If it is not, she says she and another campaigner will start judicial review proceedings at the High Court.
She said it was "disgusting" that children were being put on the drugs when they had already been banned because they were "unsafe".
In her case, the High Court ruled that under-16s were "unlikely to be able to give informed consent" to puberty blockers, but this was later overturned by the Court of Appeal which ruled that doctors can judge whether young people can give consent to the treatment.
Some clinicians from the Clinical Advisory Network on Sex and Gender, which campaigns for rigorous science and improved treatment options for gender-questioning people, have also questioned whether the trial can be carried out ethically.