BANGKOK (AFP) - Thailand's constitutional court on Tuesday (Dec 1) warned against "vulgar" criticism of its rulings, on the eve of a crucial judgment that could see Prime Minister Prayut Chan-o-cha forced out of office.

The court will on Wednesday hand down a verdict on whether Mr Prayut has broken rules by living in an army house, despite the fact that the former general is no longer in the military top brass.

Losing the case could see Mr Prayut, who came to power in a 2014 coup, thrown out as premier.

The ruling follows months of street protests calling for Mr Prayut to quit, and one pro-democracy group plans to rally outside the court on Wednesday.

But the court urged people to stay away and warned that excessive criticism could lead to prosecution.

"A person shall enjoy the liberty to express opinions, but criticism of rulings made with vulgar, sarcastic or threatening words will be considered a violation of the law," the court said in a statement.

The pro-democracy movement is already facing legal action, with five key leaders charged on Monday under Thailand's strict royal defamation laws - the first time they have been used in two years.

As well as calling for Mr Prayut to go, protesters also want reforms to the army-drafted Constitution and for changes to the monarchy - a taboo-smashing demand in a country that has long revered its royal family.

Mr Prayut previously argued his family must stay at the army house on a military base for security reasons.

He appeared relaxed on Tuesday ahead of the decision, saying he would send his lawyers to the court but would not go in person.

"I've had cases filed against me before. If the court rules that you've done wrong then you've done wrong... if not then that's the end of the story. It's just like any court case," he told reporters in Bangkok.