A new law makes criticism of Thailand's Constitutional Court punishable by imprisonment, giving extra protection to a body that in recent years has made controversial rulings altering the shape of governments.

The unheralded law took effect when it was published last week. It says honest criticism without rude, sarcastic or threatening words is not in violation of the law, but if such words are used, it is punishable by up to one month in jail and a 50,000 baht ($1,600) fine.

Defamation is a criminal offence in Thailand, punishable by up to two years in prison, but must first be filed through the police. The new law allows the Constitutional Court to deem verbal attacks a "violation of court powers," enabling it to initiate its own cases.