Police Maj. Gen. Kriangsak Arunsrisophon, commander of the Economic and Cyber Crime Division, said Tassaporn Ratwongsa, a hospital radiologist, was charged with spreading false information deemed harmful to national security, a violation of the Computer Crime Act. If found guilty, she could receive up to five years in prison and a fine of 100,000 baht ($2,770).

Tassaporn and three other suspects arrested earlier have been released on bail, Kriangsak said.

Rumors about the health of 81-year-old King Bhumibol Adulyadej sent Thailand's stock market tumbling in mid-October. The sell-off was seen as reflecting widespread concern over the issue of royal succession.

The king has been hospitalized since Sept. 19 with symptoms of pneumonia, but doctors say he is recovering.

Thailand has endured more than three years of sometimes violent political turmoil and there is worry about what effect the loss of the king would have.

The monarch has historically been the country's main unifying figure in times of crisis, even though he is a constitutional monarch with moral authority rather than legal powers. The king's 57-year-old son and heir apparent, Crown Prince Vajiralongkorn, does not yet have the moral authority or popularity of his father.

Critics have called the broadly defined Computer Crime Act, enacted in 2007, a threat to freedom of speech.

Open discussion of the monarchy is also constrained by strict lese majeste laws that make criticism punishable by up to 15 years in prison.

Lese majeste prosecutions used to be rare in Thailand, and the accusation was mostly used for partisan political purposes as a means of smearing opponents.

But in recent years, as nervousness about royal succession has increased, the number of high-profile cases has risen, with the previously taboo subject of the monarchy's proper role beginning to be a matter of public debate.