Maya Forstater, the chief executive of Sex Matters, a campaign group, said it was “grossly irresponsible” of Dr Cass “to put children at grave risk by making misleading and unevidenced remarks in support of a trial of puberty blockers”.
She said there was “no evidence” to support Dr Cass’s claims that it would reduce children accessing unregulated drugs.
“Her review was groundbreaking when it was published more than two years ago, but it has since been superseded by other systematic reviews, which were able to draw on more recent studies,” she said.
“Each successive piece of research shows more clearly that puberty blockers are a fundamentally harmful treatment that should simply not be offered to children unhappy about their sex. The law is now clear, and no child should have their body harmed in the quest for an impossible outcome.
“Around 2,000 children have already been exposed to these drugs. If further evidence is required it should be provided by finding out how they have fared.”
Under the new protocol approved by the MHRA, girls will need to be at least 11 and boys 12 to be put on puberty blockers.
The medicines regulator had previously said it was concerned about “potentially significant” biological harm and suggested a minimum age of 14 for participants. Initially, there was no minimum age beyond having started puberty, which for some girls can be as young as eight.
Puberty blockers are hormone-suppressing drugs that pause the physical changes of adulthood, such as facial hair and periods, and have been used to treat childhood gender dysphoria, when a child thinks their gender does not align with their sex.