A political activist from Thailand's Red Shirt protest movement has been sentenced to 15 years in prison for insulting the country's monarchy.

The Criminal Court in Bangkok on Thursday found Daranee Charnchoengsilpakul guilty of making several defamatory comments against the royal family during a speech at a political rally in 2008.

Known as “Da Torpedo” for her fiery speaking style, Daranee was originally sentenced to 18 years in prison before an appeals court granted her a retrial. She says she does not plan to appeal the latest ruling.

She is a member of Thailand's Red Shirt Pheu Thai party founded by exiled former Prime Minister Thaksin Shinawatra. Mr. Thaksin's sister, Yingluck Shinawatra, is now prime minister.

Her case is the latest in a series of convictions under Thailand's toughest in the world lese majeste laws, which call for a prison sentence of three to 15 years for anyone who “defames, insults, or threatens” the country's monarchy.

Last month, a 61-year-old Thai grandfather was sentenced to 20 years in prison for sending four “inappropriate” text messages to a government official.

Earlier this month, a Thai-born U.S. citizen received 30 months in prison after posting and translating parts of a banned biography of the king on his personal blog.

Human rights groups and Western leaders have urged Thailand to amend the controversial laws, saying they are being increasingly used to silence activists and politicians.

Thailand's 84-year-old King Bhumibol Adulyadej is widely revered by Thais as a unifying figure in the politically polarized nation, and talks of removing the laws have proven too contentious.