A high-profile clinical trial of drugs that can delay puberty in children questioning their gender can go ahead after the High Court rejected a legal challenge by campaigners.

The Pathways trial, run by researchers at King’s College London (KCL), will start recruiting more than 220 children aged between 11 and 16 years old from 1 August.

The study was commissioned to assess the benefits or risks of puberty blockers, after a landmark review into gender services found the evidence for treating young people with the drugs was “remarkably weak”.

Campaigners who brought the legal action had argued the safeguards in place were inadequate and raised concerns about the long-term physical and psychological effects on young people.

The judge, Mr Justice Chamberlain, refused the application for a judicial review saying that none of the claims made by the campaigners had a realistic chance of success.

He said that each patient involved in the trial will have been assessed over “many months” by specialist doctors who will have “formed the opinion that a child has a reasonable prospect of receiving a clinical benefit from the treatment”.

A spokeswoman for KCL welcomed the court’s decision, saying the “safety and welfare of young people will remain our highest priority in carrying out this research”.

Campaigners, including parents of gender-questioning children, said they were disappointed by the decision saying the “world is watching” a case which has “global implications and high stakes”.

A landmark review into young people’s gender care, written by Baroness Hilary Cass, was published in 2024.

The government then banned the use of puberty blocking drugs in children’s gender care across the UK. The drugs can pause the physical changes of puberty such as breast development and voice deepening.

Children already on the medication were allowed to continue but new prescriptions could only be offered as part of a controlled clinical trial.

NHS England commissioned the Pathways study to try to establish scientific evidence about the possible health risks and benefits.

Speaking to the BBC in June, Cass said that the trial was “essential” to answer the question about “whether these drugs are helpful or not”.

KCL researchers have said they hope to recruit 226 children under the age of 16 who are already accessing specialist gender services in England and have a diagnosis of “gender incongruence”, where a person’s sense of their gender does not match the sex they were recorded as at birth.

The team has said any young person participating in the “carefully designed study” will undergo detailed medical and psychological assessments. They and their family will also have to demonstrate they understand the potential impact of taking part.

The trial will also look at the effect of the drugs on bone density, brain development, fertility and mental wellbeing.

In late 2025, campaigners brought legal action against the government and health bodies claiming the trial was unethical and that children could not give properly informed consent.

One of those taking legal action was Bayswater Support Group, which works with 800 parents who are wary of medical solutions for their gender-questioning children.

Others include the psychotherapist James Esses and Keira Bell, who began taking puberty blockers when she was 16 before detransitioning and speaking to the media about her experience.

“It’s a tragedy that the court has refused permission for this challenge to proceed,” she said.

“The medical profession and morality do not always walk hand-in-hand. If they did, this trial would be out of the question.”

The Bayswater Group said the design and size of the study, and the lack of evidence about any possible benefits of puberty blockers, “raise fundamental questions about whether the protective framework for minors has been properly implemented”.

The Pathways trial was originally due to start recruiting participants in January 2026 but was paused because of the legal action.

When details of the trial were first announced, there was no lower age limit for involvement, but in February 2026 the medicines regulator, the MHRA, raised safety concerns and suggested a minimum age of 14 should be set.

In June it announced that, after discussions with researchers, safeguards had been strengthened and girls would be able to take part from age 11 years old with boys participating from 12.