The judges, he said, had "the responsibility to prevent the country from collapsing".
"Whatever the verdict will be, it will bring damage to the country," he said. "Whatever direction it will take, it will be erroneous."
Thailand's widely-revered king, the world's longest-reigning monarch, rarely intervenes in politics.
The tone of the speech implied he was urging the judges not to compromise in an attempt to please everyone.
"In my mind, I have a judgement but I cannot say," he said, without elaborating.
"I urge you to prepare yourself to be ready to criticise or be criticised in the capacity of learned men to prevent the country from falling into a crisis"
King Bhumibol Adulyadej
"I urge you to prepare yourself to be ready to criticise or be criticised in the capacity of learned men to prevent the country from falling into a crisis," he said.
"If you don't do anything, the country will fall."
The king's comments hinted at possible trouble from unhappy supporters of Thaksin Shinawatra, the former prime minister ousted in a military coup last September.
Thaksin's Thai Rak Thai party and its rival, the Democrat Party, are accused of violating election laws in an inconclusive general election last year that was later annulled by the courts.
The parties face dissolution and their top leadership, including Thaksin, who is living in exile in London, could be banned from politics for five years.
Al Jazeera's correspondent in Bangkok, Selina Downes, says the military has promised fresh elections in December, but who will stand in them if both parties are dissolved is unclear.
|Coup leaders have pledged to restore |
democracy by the end of the year [EPA]
Earlier this week a former deputy leader of Thai Rak Thai threatened to mobilise thousands of protesters if the court ruled against it.
The army-installed government however said it was prepared to invoke an emergency decree if demonstrations triggered by the ruling turn violent.
Al Jazeera's correspondent says police and the military have set up roadblocks across Bangkok ahead of next week’s rulings, hoping to deter protestors from descending on the city and taking part in demonstrations many fear could turn violent.
Observers say the generals behind last year's coup want the constitutional court's verdict to effectively bar Thaksin from returning to politics.
They say that if Thai Rak Thai is not disbanded the justification for ousting him on charges of abuse of power and corruption would be undermined.
The king's address came as a bill was introduced in the US congress seeking to remove Thailand's designation as a major non-NATO ally until a democratically-elected government is returned.
For Thailand the benefits of ally status largely involve the sale or loan of weapons or other defence equipment.
Mark Kirk, a Republican member of the house of representatives, said the bill, was to "set an example for other Asian militaries to stay out of politics."
"We have an obligation to stand with the Thai people to prevent future abuses by the unelected Government of Thailand," Kirk said in a statement.
"A military dictatorship that disposes an elected government should not be considered a major non-NATO ally."