Children cannot properly understand the “lifelong medical, psychological and emotional implications” of taking “experimental” puberty blockers and cross-sex hormones, the High Court has heard.

A 23-year-old woman who began taking puberty blockers when she was 16 before “detransitioning” and the mother of a 16-year-old autistic girl are suing the Tavistock and Portman NHS Trust, which runs the UK’s only gender identity development service (GIDS) for children.

Lawyers representing Keira Bell and Mrs A, whose child is currently on the waiting list for treatment at the service, argue that children going through puberty are “not capable of properly understanding the nature and effects of hormone blockers”.

They argue that there is “a very high likelihood” that children who start taking hormone blockers will later begin taking cross-sex hormones, which they say cause “irreversible changes”.

Ms Bell and Mrs A are asking the High Court to rule it is unlawful for children who wish to undergo gender reassignment to be prescribed hormone blockers without an order from the court that such treatment is in their “best interests”.

At a hearing in London on Wednesday, the pair’s barrister Jeremy Hyam QC said: “The use of hormone blockers to address gender dysphoria does not have any adequate evidence base to support it.”

He said that “the effect of hormone blockers on the intensity, duration and outcome of adolescent development is largely unknown”, adding: “There is evidence that hormone blockers can have significant side-effects, including loss of fertility and sexual function and decreased bone density.”

In written submissions, Mr Hyam said: “That children are not capable of giving informed consent to undergo a type of medical intervention about which the evidence base is poor, the risks and potential side-effects are still largely unknown, and which is likely to set them on a path towards permanent and life-altering physical, psychological, emotional and developmental consequences … is the common-sense and obvious position.”

The barrister told the court that referrals to GIDS had risen from 97 in 2009 to 2,590 in 2018, “essentially a twenty-fold increase”.

In a witness statement before the court, Ms Bell said: “I made a brash decision as a teenager, as a lot of teenagers do, trying to find confidence and happiness, except now the rest of my life will be negatively affected.”

She added: “Transition was a very temporary, superficial fix for a very complex identity issue.”

Fenella Morris QC, representing the Tavistock and Portman NHS Trust, said the contention that children could not give informed consent to being prescribed hormone blockers was “a radical proposition”.

She argued in written submissions that the claimants sought to “impose a blanket exclusion” on children under the age of 18 to be able to consent to medical treatment.

Ms Morris added the majority of children referred to GIDS between March 2019 and 2020 were over 12, with only 13 of the children referred being under the age of 13.

She accepted that hormone blockers were “experimental” but argued their use “has been widely researched and debated for three decades”, adding: “It is a safe and reversible treatment with a well-established history.”

Ms Morris also said the Tavistock and Portman NHS Trust referred children and young people experiencing gender dysphoria to University College London Hospitals NHS Foundation Trust or Leeds Teaching Hospital NHS Trust.

She argued that those two trusts, not the Tavistock and Portman NHS Trust, were “responsible” for prescribing hormone blockers to children with gender dysphoria.

Lawyers representing London Hospitals NHS Foundation Trust and Leeds Teaching Hospital NHS Trust are expected to address the court on Thursday.

The hearing before Dame Victoria Sharp, Mr Justice Lewis and Mrs Justice Lieven is expected to last two days and it is expected that the court will reserve its judgment to a later date.