Dozens of transgender teenagers are freezing their sperm or eggs on the NHS in case they want children after changing sex, doctors have said.

Boys as young as 12 who have been diagnosed with “gender dysporia” are having sperm frozen before they start hormone treatment that halts the onsent of puberty.

Doctors behind the scheme, which also preserves eggs from girls who believe they were born the wrong sex, said it was only fair to give patients the right to start a family.

Professor Gary Butler, head clinician for Britain’s only NHS service for young people with gender dysphoria – the belief a person is inhabiting the wrong sex – said: “Transgender people are not making a lifestyle choice. They are following their biological and psychological make-up about their identity.