Teachers have been told in a training session that some children as young as 12 who question their gender identity should be offered hormones to block puberty.

In a meeting with 20 teachers and pastoral staff at Newman University in Birmingham last December, a trainer for the transgender lobby group Mermaids said giving puberty blockers to gender-dysphoric children gave them “immense relief” and was “completely reversible”.

The trainer, recorded by an audience member, said: “Puberty blocker medication doesn’t make any changes, so [is] completely reversible. What it does is put a pause button on the pituitary gland and freezes puberty where it is. Not growth, just puberty. Take the blockers away and biological puberty will recommence.” She added that using blockers “does provide immense relief from the dysphoria” and argued that arrested development meant less need for physical intervention later “if they decide to go down that path”.

This weekend the advice was attacked as “misleading and potentially dangerous” by an Oxford academic. The session is believed to be a blueprint for training by the group in schools nationally.

Michael Biggs, associate professor in sociology at St Cross College, Oxford, says the guidance could push children towards medical intervention. He cites research which found that most children prescribed blockers at a young age progressed to taking cross-sex hormones and surgery, which damage fertility.

Biggs says he has unpublished evidence that after a year on blockers “children experienced increased psychological problems, including self harm. Mermaids is not giving pupils a choice that they may just be gay or lesbian. Instead, it is diverted down a drastic path that ultimately ends in surgery. To be propagating such information to schools is disturbing.”

Michael Conroy, who taped the footage and has worked in pastoral support in schools for 15 years, said he believed Mermaids was encouraging young people to think that they were “born in the wrong body . . . It’s putting children at risk.”

Mermaids said: “The information in our sessions is based on a wealth of international scientific study and 25 years of experience in this field. Since this training session in 2018, we have been funded by the Department for Education in a schools training programme overseen by the Government Equalities Office.”

Last week a group of parents announced a lobbying organisation aimed at the NHS. One of them, a mother, told a public meeting that the gender reassignment of young people with autism or psychological problems was “state-sponsored sterilisation of the mentally disabled”. Members of the group attended a governors’ council of the Tavistock and Portman NHS Trust, which includes the Gender Identity Development Clinic for trans children. The mother said her 20-year-old, who identifies as a gay trans man and lives with other young people awaiting reassignment surgery, had ADHD with autistic traits diagnosed at the age of six.

She said her child fitted the description of “rapid onset gender dysphoria”, whereby youngsters declare, often in clusters, that they are trans after exposure with their peers to suggestions they are “trapped in the wrong body”.