BANGKOK (AFP) - A Thai man was sentenced to more than three years in jail on Thursday under the kingdom's controversial royal defamation law, despite having a history of mental illness.

Thanet Nonthakot is the second person in the last month suffering from a mental health condition to be convicted under Thailand's notorious lese majeste legislation - one of the world's strictest.

Under Section 112 of Thailand's criminal code anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

Thanet, 45, from northeastern Phetchabun province, was found guilty of insulting Thailand's monarchy over the contents of an email he sent four years ago, a judge at Bangkok's main criminal court said.

A psychologist at his trial testified that Thanet suffered from a mental health illness but was fit to stand trial and was aware of his actions when the alleged offence took place.

"The defendant cannot prove that he was unable to control himself when he sent the email," the judge said.

Thanet denied breaching the royal defamation law but accepted he had sent the email.

He was handed a five years jail term but the sentence was reduced to three years and four months thanks to his "useful testimony".

Local and foreign media must heavily self-censor when reporting lese majeste cases. Even repeating details of the charges could mean breaking the law.

Lese majeste prosecutions have surged since former army chief Prayut Chan-O-Cha seized power in May 2014.

According to iLaw, a local rights group that monitors such cases, just two people were being prosecuted for royal defamation before the coup. Now that number is at least 46.

Recent cases include a 58-year-old man sentenced to 25 years in prison for the content of five Facebook posts and a bookseller jailed for an alleged offence back in 2006.

In early May a mentally ill 65-year-old woman was also jailed for allegedly insulting a portrait of King Bhumibol Adulyadej.

Critics of the law say it is frequently used to pursue political opponents of the royalist elite and their military allies.

Public discussion of the law, its impact and application is virtually impossible. Earlier this month the junta banned the Foreign Correspondents Club of Thailand from holding a planned debate on the law.

On 19 June the United Nations' Committee on Economic, Social and Cultural Rights criticised the "excessive interpretation of lese majeste" saying it had an "adverse effect" on Thai society.

"The abuse of Article 112 has become an international embarrassment for Thailand and it is damaging the image of the Thai monarchy," Mr Karim Lahidji, president of the International Federation of Human Rights said in a statement published on Tuesday in response to the UN criticism.

"It's time for Thailand to heed the numerous recommendations made by various UN human rights bodies to amend its lese-majeste legislation," he urged.