He said Mr. Samak’s cabinet was to remain in place as a caretaker government until Parliament named a new prime minister.

A hardened and sharp-tongued politician, Mr. Samak, 73, has shown a folksy side on his televised cooking show — "Tasting and Complaining" — stirring up personal recipes and sounding off on topics that would catch his interest.

"I have done nothing wrong," the prime minister told the Constitutional court on Monday. "I was hired to appear on the program and got paid from time to time. I was not an employee of the company."

He said, "I did it because I liked doing it."

Mr. Samak made just a few appearances on the show after becoming prime minister seven months ago, but that was enough for a group of anti-government senators, who brought his case to the Counter Corruption Commission, which forwarded it to the Constitutional Court.

The courts in Thailand have become an active part of the political scene, and have charged former Prime Minister Thaksin Shinawatra in several corruption cases.

Mr. Thaksin fled to London last month to escape his court dates and has applied there for political asylum.

Mr. Samak also faces three charges of corruption that have not yet reached the courts, and he is appealing a three-year prison sentence for defamation of Bangkok’s deputy governor, Samart Rapholasit.

Conviction in any of these cases would also force him to step down. In addition, another independent agency, the Election Commission, ruled last week that Mr. Samak’s party, the People Power Party, had committed electoral fraud in an election last December and should be dissolved. That case too is expected to be heard by the Constitutional Court.

In the face of these mounting difficulties, Mr. Samak has refused calls to resign and declared a state of emergency following an outbreak of violence a week ago in which one demonstrator was killed and several wounded.

But the military has said it would not use force to carry out his decree, taking what it calls a neutral stance between the demonstrators and the government.

The verdict was an unexpected twist in a long-running crisis that began in late 2005 when the People’s Alliance for Democracy launched protests against Thaksin’s government. The protests led to a non-violent military coup in September 2006 while Mr. Thaksin was abroad, and Thailand was ruled for more than a year by a military-backed government.

When the military relinquished power in a parliamentary election last December, Mr. Thaksin’s supporters won a large majority in Parliament and named Mr. Samak as prime minister.

Mr. Samak openly declared himself a nominee for Mr. Thaksin, and the protests against Mr. Thaksin and his group resumed last May, with the protesters consisting of an odd-fellow makeup of royalists, bureaucrats, military men, union leaders and some liberal democrats.

The People’s Alliance for Democracy is calling for an overhaul of Thailand’s political system that would weaken electoral politics with a mostly appointed Parliament.

Mr. Samak says he has democracy on his side after an overwhelming electoral victory that gave his coalition close to two thirds of the seats in Parliament.