'As of today, doctors can perform the surgery if there is a medical reason to do so - not for any other reason,' ministry spokesman Suphan Srithamma said.

A letter will be sent to medical facilities around the country telling them to halt so-called commercial castrations until further notice, he said. Any violators could face closure of their practices.

Mr Suphan said he did not have official statistics on the numbers of castrations performed in Thailand, but said many underage patients were unaware of the risks it posed, including hormonal imbalances and stunted physical development.

The ministry and the Medical Council of Thailand will draft new guidelines that doctors must follow before carrying out the procedures.

Existing rules require boys under age 18 to have parental consent before undergoing castration.

The ministry's move came after a leading gay activist, Mr Natee Teerarojjanapongs, called on the Medical Council to take action against clinics that perform castrations on underage boys.

Mr Natee, head of the Gay Political Group of Thailand, said he had received several complaints from parents of underage boys seeking castrations in part because of widespread online ads that promise cheap operations resulting in feminine qualities like softer skin.

Clinics around Bangkok charge for about US$100 (S$138) for the procedure.

'It's a totally wrong perception that castration will make boys more feminine,' Mr Natee was quoted as saying last week by The Bangkok Post. 'These youngsters should wait until they are mature enough to thoroughly consider the pros and cons of such an operation.' - AP