A former transgender patient has mounted an unprecedented legal challenge against a sex change clinic she claims is putting children on a "torturous”, “permanent” and “unnecessary path, High Court documents reveal.

The Tavistock and Portman NHS Trust, which runs the UK's only gender identity development service (GIDS) for children, is being sued over concerns that youngsters are being given "experimental treatment" without adequate assessments.

The landmark case centres around a bid to stop the NHS prescribing "experimental" puberty blockers and cross-sex hormones to children who wish to undergo gender reassignment.

The case is being brought by the former patient alongside a woman known only as Mrs A, the mother of a 15-year-old autistic girl who is currently on the waiting list for treatment at the service.

Earlier this month The Telegraph reported that Susan Evans, 62, a former psychiatric nurse at the Trust, was the lead claimant in the case along with Mrs A. She said that “experimental” and “invasive medical treatment” should be prevented in order to protect children.

However it can now be revealed that Keira Bell, a former Tavistock patient who is unemployed and lives in the Cambridge area, is taking her place as the lead claimant in the High Court challenge.