The World Professional Association for Transgender Health (WPATH) is proposing that children as young as 14 should be given transitioning treatments, including surgery to “masculinise” girl’s chests when they reach the age of 15.

Under NHS guidelines, “cross-sex” hormones that start a physical transition can only be given from 16 and surgery is not allowed until 18. But WPATH – whose recommendations are in a set of draft “standards” put out for consultation – believes this is too late and may cause “lifelong harm”.

The proposals represent a big change in how the US-based organisation thinks “gender diverse” teens should be treated. Until now it has emphasised the use of puberty-blocker drugs to freeze physical development, but it is advocating moving sooner to irreversible cross-sex hormones and surgery.

Among WPATH’s arguments for lowering the ages are concerns over the potential long-term side effects of puberty blockers, such as weaker bones in adulthood and “potential neurodevelopmental” problems. The organisation has previously waved away such concerns and insisted that blockers are “fully reversible”.

The group also cites “emerging evidence” that “early medical intervention ... can be effective” for teenagers with gender identity issues, but accepts the number of studies is limited, involved only a small number of participants and focused on those who started questioning their gender when they were young children.

Binary spokeswoman, Kirralie Smith, said the recommendations are dangerous.

“It is outrageous to experiment on children. There are not enough long term studies on these treatments and most are irreversible,” she said.

“Children cannot get tattoos, drink alcohol or drive a vehicle at these ages because of the lifelong consequences of such decisions.

“Nor should they be able to access medical or surgical interventions until they are adults.”