The Bangkok court on Wednesday unanimously ruled that Prime Minister Yingluck in 2011 improperly reassigned her then national security chief. It said the move was unconstitutional and done for her own benefit.

Several members of Ms. Yingluck's cabinet found to be complicit in the decision were also ordered to be removed from office.

It is not clear whether a deputy prime minister or other official would replace Ms. Yingluck as the acting head of Thailand's caretaker government or whether the verdict would create a power vacuum.

Prime ministers are usually chosen by the country's lower house of parliament, but that body was dissolved by Ms. Yingluck last year when she called for early elections in an attempt to resolve the crisis.

Ms. Yingluck, who was Thailand's first female prime minister, defended herself before the court on Tuesday. She has not commented on the ruling.

The prime minister's supporters say the charges are politically motivated. They had threatened to protest if she were removed, raising fears of violence. About two dozen people have already been killed in six months of anti-government protests.

After the protesters failed to achieve their goal of ousting Ms. Yingluck, they instead turned to the Constitutional Court, which has a history of ruling against Shinawatra-linked governments.

The opposition protesters say Ms. Yingluck's government is hopelessly corrupt and controlled by her brother, ex-prime minister Thaksin Shinawatra.

Thaksin was removed from office in a 2006 military coup. The billionaire businessman is still very influential in Thailand. He is living in exile to escape corruption charges.

The conflict pits Bangkok's urban middle class and royalist elite against the mostly poor, rural supporters of Ms. Yingluck and Thaksin.