Parents are risking psychologically damaging their children by allowing them to “socially transition” their gender without medical or psychiatric advice, NHS experts have warned.

Primary school-aged children are increasingly being encouraged to formally switch, in defiance of the recommended “watchful waiting” approach, the Gender Identity Development Service (GIDS) leaders said.

In some cases, children as young as six are attending school where nobody knows their original sex.

In the UK, children who display symptoms of gender dysphoria are not given hormone blockers until the onset of puberty, and cross-sex hormones may only be prescribed after they turn 16.

The GIDS psychologists, who practise at London’s Tavistock Clinic, said that younger children who believe they may have been born with the wrong body should be permitted to explore behavioural aspects of the opposite gender, such as dress or types of play.

However, they warned that many such children end up preferring to remain the biological gender they were born, and that to formally socially transition before puberty risks pre-determining the outcome.

They acknowledged that well-meaning parents, faced with deeply unhappy children, can sometimes feel they have no other option.

The situation is made worse because the waiting list to see a specialist at the Tavistock and Portland Trust, the NHS’s only child gender service, is now two years’ long.