A landmark test case to establish whether children can give informed consent to medical treatment for gender reassignment begins in the high court this week.

Lawyers acting for Susan Evans, a former psychiatric nurse at the Tavistock and Portman NHS foundation trust, which runs the UK’s only NHS gender identity development service (Gids), and “Mrs A”, the mother of an autistic 15-year-old girl who is on the Gids waiting list, will file papers to commence proceedings in a judicial review brought against the trust and NHS England.

At the heart of the case is the provision of puberty blockers and cross-sex hormones to young people who wish to transition or are considering doing so.

“We are essentially seeking to say that the provision at the Tavistock for young people up to the age of 18 is illegal because there isn’t valid consent,” said Paul Conrathe, a solicitor with Sinclairslaw, which is representing Evans and the mother.

Providing this treatment – puberty blocking and cross-sex hormones – to any young person who wants them requires, he argues, “a specific order of the court on a case-by-case basis. [The treatment] cannot be delivered as a matter of general approach”. Conrathe suggested the legal action would be “pressing the case of Gillick to its breaking point”.

In 1983, Victoria Gillick, a Roman Catholic mother of 10, challenged the right of doctors to prescribe contraception to girls under the age of 16 without their parents’ permission or even knowledge. Two years later the House of Lords affirmed the doctors’ right, ruling that “the parental right to determine whether or not their minor child below the age of 16 will have medical treatment terminates if and when the child achieves sufficient understanding and intelligence to fully understand what is proposed”.

But Conrathe said the “Gillick competence” test should not apply when it comes to gender reassignment: “The issue is whether the young person is of sufficient maturity and capacity to understand the consequences of their actions. We say it is a leap too far to think that Gillick as a judgment could apply to this type of scenario, where a young person is being offered a treatment with lifelong consequences when they are at a stage of emotional and mental vulnerability. It simply doesn’t compute, and therefore whatever medical professionals say is consent is not valid in law.”

He acknowledged that the case would break new ground. “I don’t think there has been any case that has tested a policy or practice in this way. There may have been the odd case that has come up but not one that has challenged a health service for making this service available.”

According to her crowdfunding page on the CrowdJustice website, Evans raised concerns about the treatment approach of the Tavistock with its clinical management team.

“The alarm bells began ringing for me when a colleague at the weekly team clinical meeting said that they had seen a young person four times and they were now recommending them for a referral to the endocrinology department to commence hormone therapy,” she writes.

Her actions triggered an internal inquiry in 2004 but Evans left the trust as she felt “nothing really changed”. Her husband, Marcus, resigned as a governor of the trust last February, accusing its management of having an “overvalued belief” in the expertise of the Gids that was “used to dismiss challenge and examination”.

On the CrowdJustice website, Mrs A, the mother of the autistic teenager, said she worried that “no one (let alone my daughter) understands the risks and therefore cannot ensure informed consent is obtained”.

NHS England said it would not comment ahead of the hearing. A spokeswoman for the Tavistock and Portman NHS 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 monitor our service very closely. The service has a high level of reported satisfaction and was rated good by the Care Quality Commission.”