However, bodily and hormonal changes will continue as normal if the medication is stopped.

Until this month, British doctors were prevented from offering youngsters diagnosed with gender issues any medical intervention before the late stages of puberty usually at 16.

But now the National Research Ethics Service has given approval to the UK's only specialist clinic for GID – the Tavistock and Portman NHS Trust in London – to prescribe the drugs to youngsters from 12 years old.

Dr Polly Carmichael, the clinic director, said the reduction in the age limit will be welcomed by families who would have otherwise had to travel abroad to the US for the treatment.

"The majority of our referrals are 15-plus and we get fewer from a younger age group," she said.

"Certainly, of the children between 12 and 14, there's a number who are keen to take part.

"I know what's been very hard for their families is knowing that there's something available but it's not available here.

"This delay gives us a window to explore together that they are definitely making the right decision But as professionals we need to be looking at the long term and making sure this treatment is safe."

The hormone blockers, which are already used for early onset puberty, will only be given to around a dozen children and teenagers selected for an NHS research project jointly run by the clinic and University College London Hospital.

To take part, they will have to meet strict eligibility criteria including having full support from their parents, the existence of long-standing gender identity issues, an ability by the child to give formal consent and an absence of other mental health problems.

Those chosen will go through a series of psychological and medical assessments before receiving the blockers.

As a result, they will stop producing the hormones which tell the sex glands to make oestrogen or testosterone.

Boys will be prevented from developing male traits such as facial hair, deeper voices and Adam's apples and girls will not develop breasts or menstruate.

But Dr Carmichael said that only around 10 to 20 per cent of prepubescent children with GID went onto have a sex change.

Around 80 per cent in late puberty were likely to have the operation.

The Tavistock and Portman clinic is currently the only place in Britain where under-18s with GID and their relatives can access specialist psychotherapy.

The 22-year-old service, which received 127 GID referrals this year, helps families avoid common traps such the temptation to keep their child's condition secret.