The drugs, taken as injections, give under 16s with Gender Identity Disorder (GID), time to decide which sex they feel they should be by delaying the onset of puberty through suppressing sex hormones.

People with GID believe their gender identity is different to their biological sex.

The drugs would delay the development of facial hair or breasts. Children would only be administered the drugs after being subject to mental health assessments.

Until now, under-16s in Britain seeking treatment for GID, had the choice of going overseas or waiting until the age of 16 to proceed with gender reassignment surgery.

The injections are being trialled at a North London clinic run by the Tavistock and Portman NHS Foundation.

Dr Polly Carmichael, director of Tavistock’s gender identity development service (GIDS), said the treatment provides a “space for ongoing therapeutic exploration. If the gender dysphoria persists some young people may decide to move on to taking cross-sex hormones.”

She adds: “A positive benefit of earlier intervention for this group is that the development of irreversible physical secondary sex characteristics in the unwanted gender will have been halted – which makes transitioning easier and should improve long term outcomes in terms of the body matching the gender identity.”

The number of people diagnosed with GID trebled between 1998 and 2010 to 12,500, according to a 2011 report by the Gender Identity Research and Education Society (Gires).

Gires Trustee, Bernard Reed, OBE, an advocate of the new treatment, said: “The internet and online forums have been a major factor in spreading knowledge and giving sufferers the confidence to seek help.”

He added: “We have been campaigning since 2005 to bring Britain in line with other countries, such as Belgium and Canada, by offering children the opportunity to suspend the process of puberty while they make a decision.”

Families and children affected By Gender Identity Disorder can seek advice and support through Mermaids. Information is also available for families on the Gender Identity Research and Education Society website.