Paul Conrathe, the lawyer who is representing Ms Bell and the two other individuals, said: “Given what we know about the unscrupulous and harmful practices of unregulated overseas gender clinics it is remarkable that the secretary of state has not banned the prescription of cross-sex hormones.
“He banned puberty blockers, but failed to ban even more harmful cross-sex hormones that expose children to irreversible lifelong changes.
“This harmful trade continues unashamed and emboldened at the failure of the government to protect vulnerable children.”
The Department of Health and Social Care did not respond directly to the threat of a judicial review, but said it was working with regulators to tighten up rules around private provision of cross-sex hormones.
A spokeswoman pointed out that the landmark Cass Review into gender care published last year did not call for a ban of cross-sex hormones for those aged 16 to 18.
She added that “additional safeguards” were being put in place in the NHS with a group of health professionals now reviewing cases before cross-sex hormones are given to young people.
The NHS can prescribe cross-sex hormones to under 18s.
But the Cass Review found that there was a lack of research and “weak evidence” for medical interventions.
As a result, NHS England wrote to all NHS gender clinics saying “extreme caution” should be exercised before recommending cross-sex hormones for under 18s.
Changes are being made to the way NHS gender services for children are run with a move away from medical interventions and a greater focus on wider needs, including with mental health. Puberty blockers are no longer used by them at all – except in clinical trials.
The legal move comes as the courts continue to hear the case involving a mother who is trying to stop her 16-year-old being given cross-sex hormones to change their gender.
The case will return to the High Court on Friday.
Between the ages of 16 and 18, whilst not legally an adult, a young person is usually viewed as being able to make their own medical decisions.