Dozens of transgender teenagers are freezing their sperm or preserving their eggs on the NHS so that they can have babies after changing sex, The Mail on Sunday can reveal.

Adolescent boys – some as young as 12 – who believe they are female are having their sperm frozen before receiving powerful hormone treatment that halts the onset of puberty and shuts down their reproductive system.

They then have the option to use the frozen sperm so that they can father their own biological children after having sex-change surgery.

Adolescent boys – some as young as 12 – who believe they are female are having their sperm frozen before receiving powerful hormone treatment that halts the onset of puberty and shuts down their reproductive system

NHS clinics are also freezing the eggs of girls from the age of 16 before they begin taking male hormones that will reduce fertility. These eggs can then be used at a later date so they too can have their own babies.

Egg-freezing costs £4,000 for one cycle and around £300 for storage a year, while the price of preserving sperm is around £400, plus £300 a year to keep it frozen.

The current cost of providing fertility treatment to transgender teenagers on the NHS is believed to run into hundreds of thousands of pounds. But this bill is expected to rise as more young people say they have been born the wrong sex.

Last night, critics said cash-strapped health authorities should not be spending such large sums on helping transgender patients have babies when basic services such as cataract operations and hip replacements are being rationed.

Bishop Michael Nazir-Ali, former chairman of the ethics committee of the Human Fertilisation and Embryology Authority, said: ‘The NHS is about treating people who are ill – that’s what we pay our taxes for. It is not to aid people’s various wishes about what they want to do with their bodies or their futures.

‘With increasing pressure on the NHS and so many essential services not being delivered, where are these funds for fertility treatment coming from?’

But one leading doctor who treats transgender teenagers said it was only fair to offer the egg and sperm-freezing treatment on the NHS because they have ‘the right’ to start a family.

The teenage patients are offered the chance to preserve their fertility before beginning their transition to girls by taking hormone injections

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: ‘There needs to be national guidelines to allow equal access to fertility treatment across the country for transpatients. Transgender people are not making a lifestyle choice. They are following their biological and psychological make-up about their identity.

‘If they want to become parents and raise a family the science and medicine is there. It’s the right of the individual to be able to do that.’

Prof Butler said dozens of ‘trans girls’ – those born male – who attend his Gender Identity Development (GID) NHS clinic for under-18s at London’s University College Hospital are freezing their sperm.

They are referred to his service after undergoing detailed psychological assessments at a nearby sister service called the Tavistock Clinic.

The teenage patients are offered the chance to preserve their fertility before beginning their transition to girls by taking hormone injections, known as ‘puberty-blockers’, that temporarily stop them producing sperm.

Earlier this year, The Mail on Sunday revealed that 800 children and teenagers – some as young as ten – were receiving the controversial puberty-blocking jabs.