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A former female-to-male transgender who "detransitioned" back to a woman won a groundbreaking lawsuit against a transgender clinic that failed to carry out a proper mental assessment before prescribing puberty blockers for her as a teen.

Keira Bell, 23, brought legal action against Tavistock and Portman NHS Trust, the only clinic in the UK that provides gender identity development services for transgender children.

Judges presiding over Keira's case said doctors at the Trust must now get a court order before prescribing puberty blockers for children under age 16.

The judges said children are unable to mentally understand the complex consequences of puberty blockers and cross-sex drugs.

"Children under 16 who wish to undergo under reassignment can only consent to having puberty blockers if they are able to understand the nature of the treatment," the high Court ruled in its landmark decision.

Keira, who was prescribed puberty blockers at age 16, was injected with testosterone at 17 and had her breasts removed at age 20. She is now living as a woman.

She said she felt like a "guinea pig" and she claimed the doctors at the Trust failed to perform a proper psychiatric assessment on her before she transitioned.

Trans children's charity group Mermaids called the ruling a "devastating blow" for trans children waiting for sex change treatment.