BANGKOK (AFP) - A lawmaker from Thailand's toppled ruling party was jailed for 2-1/2 years on Wednesday for defaming the royal family, the latest in a series of such convictions since the army grabbed power.

Prasit Chaisrisa, 49, a former MP for the Peau Thai party, confessed to committing lese majeste during a speech to "red-shirt" supporters of the then government, shortly before it was toppled in a May coup. "The judge initially sentenced him to five years in prison but halved the term because he confessed," a Bangkok Criminal Court official told AFP.

Prasit, a former lawmaker for the impoverished northeastern province of Surin, was arrested just days after the May 22 coup and initially denied the charge. He changed his plea in hopes of a lighter sentence. "The suspect is a two-time MP and must be... more prudent than ordinary people before speaking," the judge said, giving his ruling.

His comments can not be reported as they would be in breach of Thailand's lese majeste law - one of the world's toughest - that shields the king, queen, heir or regent. Anyone found to have defamed, insulted or threatened them faces up to 15 years in jail for each count. Thai media outlets frequently censor themselves to avoid falling foul of the law.

Since ousting the elected government, junta leader Prayut Chan-o-cha has repeatedly vowed to crack down on anyone who insults the monarchy, which attracts deep loyalty among the military and Bangkok-based establishment.

The pledge comes as anxiety mounts over the future of the kingdom as the decades-long reign of revered but ailing King Bhumibol Adulaydej, who turns 87 on Friday, enters its twilight.

A recent study by the Paris-based International Federation of Human Rights said lese majeste cases have surged since the coup, with 18 new arrests made and outstanding cases fast-tracked through the courts.

Critics say the law, Section 112 of the Thai criminal code, stifles free speech and is encouraging a witch-hunt among Thailand's politically polarised society.

On Nov 18, a radio show host was jailed for five years by a military court for breaching the law.

A few days earlier, a 24-year-old student was jailed for 21/2 years after pleading guilty to defaming the monarchy in a message posted on Facebook under a pseudonym.

In August, a 28-year-old musician was sentenced to 15 years' jail for writing insulting Facebook posts about the monarchy between 2010 and 2011.