Thailand's health chiefs barred hospitals and clinics on Wednesday from castrating would-be "ladyboys" amid growing concern about the operation being seen as a cheap and quick alternative to a full sex-change.

In a letter to 16,000 private health units, the Public Health Ministry said doctors performing the operation outside formal sex-change therapy -- which requires rigorous physical and mental evaluation of the patient -- faced up to six months in jail.

However, senior health official Tara Chinakarn admitted that policing the temporary ban might be difficult as cosmetic removal of the testicles was such a quick operation and easy to conduct in secret.

"It's hard to track them down as it takes only 15-20 minutes to have the surgery," Tara told Reuters.

Thailand is home to a large number of "ladyboys," or "katoey" in Thai, a term that covers anything from a transvestite to a man who has undergone a full sex change.