One prominent critic said he was 'deeply disappointed' by the ruling and confirmed he was considering an appeal
Children as young as 11 can now be recruited into the NHS-backed puberty blocker trial after the High Court refused an attempt to block the controversial study.
The ruling means researchers can begin enrolling youngsters into the PATHWAYS trial after campaigners failed to convince a judge that regulators acted unlawfully in approving the research.
Around 226 children are expected to take part in the NHS-funded study, which aims to gather evidence on the effects of puberty blockers in young people experiencing gender distress.
The decision follows a two-day High Court hearing in which campaigners sought both permission for a judicial review and an emergency injunction to stop recruitment while the case was heard.
But Mr Justice Chamberlain refused permission for the judicial review to proceed, allowing the trial to move ahead.
The study had already been delayed earlier this year after the Medicines and Healthcare products Regulatory Agency (MHRA) paused recruitment over concerns about "potentially significant" biological harm before approving revised protocols in June.
James Esses, psychotherapist and one of the claimants, said he was "deeply disappointed" by the ruling and confirmed he was considering an appeal.
He said: "This judgment does not negate the serious and avoidable harm that will be caused if this trial is allowed to proceed.
"Vulnerable children should never be used as guinea pigs in medical experimentation that may lead them down a path towards irreversible physiological and emotional damage, including infertility."
Detransitioner Keira Bell, who also brought the case, described the ruling as "a tragedy", adding: "The protection of children should be an inviolate principle, but this clinical trial tells a different story."
The PATHWAYS study was commissioned following the independent Cass Review, which concluded the evidence supporting puberty blockers for children with gender distress was "remarkably weak" and recommended a properly designed clinical trial.
Routine NHS prescribing of puberty blockers for under-18s has since been banned outside clinical trials after the Commission on Human Medicines concluded the drugs posed an "unacceptable safety risk".
Ministers, however, allowed an exception for the PATHWAYS study, arguing it is needed to answer longstanding questions about the drugs' benefits and risks.
During the hearing, the claimants argued regulators approved the trial without first demonstrating there was a reasonable likelihood that the benefits to participating children would justify the risks, as required under the regulations governing clinical trials involving minors.
They also argued members of the ethics committee were not given all the expert evidence raising concerns about the study before granting approval, questioned the scientific design of the trial because every participant will eventually receive puberty blockers, and claimed children could spend years on the drugs without any clear route to further treatment after NHS England paused routine prescribing of cross-sex hormones for under-18s pending a separate review.
Dr Louise Irvine, chair of the Clinical Advisory Network on Sex and Gender, said the group believed the study should never have been approved.
She said: "The Clinical Advisory Network on Sex and Gender is extremely disappointed in today's decision not to permit judicial review of the PATHWAYS puberty blocker trial. Clinical trial regulations demand that the potential benefits must outweigh the risks."
She added: "We believe PATHWAYS falls at the first hurdle: the potential benefits do not outweigh the risks. Many risks are very serious and have been minimised or disregarded by both the trial sponsors and the regulators. We believe the regulators have failed in their primary duty of protecting children from harm."
A spokesman for the Bayswater Support Group said the organisation remained convinced the trial "does not meet the legal and ethical thresholds required for research on children" and confirmed it would explore "every remaining avenue" with its legal team, including a possible appeal.
The Government and regulators have maintained throughout that the approvals were lawful and that a carefully monitored clinical trial is needed because uncertainty remains over the long-term benefits and risks of puberty blockers.
Researchers behind PATHWAYS have previously said participant safety is their overriding priority and that the study was strengthened following discussions with the MHRA, including introducing new minimum age requirements before recruitment resumed.
