A landmark case to stop the NHS prescribing “experimental” puberty blockers and cross-sex hormones to children who wish to undergo gender reassignment has reached the high court.

The Tavistock and Portman NHS foundation trust, which runs the UK’s only gender identity development service (Gids) for children, is being sued over concerns that young people are being given “experimental treatment” without adequate assessments.

The case is being brought by a woman known only as Mrs A, the mother of a 15-year-old autistic girl who is on the waiting list for treatment at the service.

At a hearing in London on Wednesday, Mrs A’s barrister, Jeremy Hyam QC, said: “What is challenged is the current and continuing practice of the defendant ... to prescribe puberty-suppressing hormone blockers and then subsequently cross-sex hormones to children under the age of 18.”

The nature of the treatment was a serious intervention described by the defendant as having consequences that may be significant and life-changing, he said.

“That treatment is given to children – not just under the age of 16, but under the age of 12 – on the basis that those children themselves consent to the treatment and gave fully informed consent to that treatment, even though the nature of the treatment has side-effects which, we say, supported by the evidence, they cannot properly take into account.”

Hyam submitted that the way informed consent was obtained from children was “materially misleading”, adding that Tavistock “omit, for example, to explain that nearly 100% of all children who commence hormone blockers go on to take the irreversible cross-sex hormones”.

Writing on a crowdfunding page, which has so far raised more than £35,000 to cover the costs of the legal action, Mrs A said: “I have deep concerns that the current clinical approach at GIDS means that my daughter will be subjected to an experimental treatment path that is not adequately regulated, where there are insufficient safeguards, where her autism will not be properly accounted for and where no one (let alone my daughter) understands the risks and therefore cannot ensure informed consent is obtained.”

The case was also originally brought on behalf of Susan Evans, 62, who was previously employed by Tavistock as a psychiatric nurse.

However, Hyam asked the court for permission for Evans’s place as a claimant in the case to be taken by Keira Bell, a 23-year-old who was previously treated at the service.

Hyam said Bell “underwent the treatment that is in issue in the proceedings … She now very seriously regrets the process and feels that the way it was handled and her involvement in it was not appropriate and her approach is to say, essentially, this should not happen to anyone else.”

He said Evans commenced the claim and continued to support it, but that she now wished Bell to be her substitute in the case.

Mr Justice Supperstone ruled that Bell should be made a claimant in the case.

Bell said after the hearing: “I have become a claimant in this case because I do not believe that children and young people can consent to the use of powerful and experimental hormone drugs like I did.

“I believe that the current affirmative system put in place by Tavistock is inadequate as it does not allow for exploration of these gender dysphoric feelings, nor does it seek to find the underlying causes of this condition.

“Hormone-changing drugs and surgery does not work for everyone and it certainly should not be offered to someone under the age of 18 when they are emotionally and mentally vulnerable.

“The treatment urgently needs to change so that it does not put young people, like me, on a torturous and unnecessary path that is permanent and life-changing.”

In a statement after the claim was filed this month, a spokesman for the trust said: “It is not appropriate for us to comment in detail in advance of any proposed legal proceedings. The Gids is one of the longest-established services of its type in the world, with an international reputation for being cautious and considered.

“Our clinical interventions are laid out in nationally-set service specifications. NHS England monitors our service very closely. The service has a high level of reported satisfaction and was rated ‘good’ by the Care Quality Commission.”