Health Secretary Wes Streeting is being threatened with legal action if he does not ban the private sale of cross-sex hormones such as testosterone to under 18s.

Lawyers acting for three people – including campaigner Keira Bell – have written to Streeting urging him to introduce a ban as he has done for puberty blockers for children questioning their gender identity.

Ms Bell says her life has been “flipped upside down” by taking the drugs that have some irreversible effects. The other two people involved are parents and wish to remain anonymous.

Cross-sex hormone therapy is used to help people transition to a different gender from the one they were born into.

In December the health secretary announced an indefinite ban on puberty-suppressing drugs for gender dysphoria.

But in a letter sent by Sinclairs Law, Streeting has been told the law firm will seek a judicial review – a way of challenging the lawfulness of a decision by a public body – unless he follows suit with cross-sex hormones.

The lawyers said private clinics are providing these to under 18s, despite the fact that they pose at least an equal risk as puberty blockers.

The letter, seen by the BBC, points to potential health risks to children’s developing brains, bones and reproductive system.

Keira Bell has previously taken legal action against the NHS – after her treatment when she was a teenager more than a decade ago at what was then the only NHS gender clinic.

She was given puberty blockers at 16 and then the male hormone testosterone at 17 before having surgery to remove her breasts.

Ms Bell, 28, told BBC News: “I was very vulnerable and was dealing with a lot of issues.

“With cross-sex hormones… it changes not only your physical and mental state, but also your social state, especially for females who are taking testosterone. I’m one of those people.”

The hormones can cause irreversible effects, such as voice deepening.

She said: “My whole life has been flipped upside down and there’s no way back from it.”

Ms Bell says she wants to protect other young people from going through similar regret.

“Things can feel very urgent so it is important for young people to remember that your whole life is ahead of you.

“This sort of protection needs to be put into law,” she says.

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.