Thailand's attorney general's office on Wednesday said former Thai prime minister Thaksin Shinawatra would be prosecuted for insulting the monarchy over comments he made almost a decade ago.

Prayuth Pecharakun, spokesman for the attorney general, said Thaksin would be summoned to court on June 18 to answer charges under the kingdom's strict lese-majeste laws.

Thaksin, 74, is a two-time premier who was ousted in a 2006 coup and then lived in self exile for 15 years.

He returned to Thailand last year as his Pheu Thai party took power at the head of a coalition government.

"The attorney general has decided to indict Thaksin for insulting the monarchy," Prayuth told reporters.

"The attorney general cannot bring him to court today, as his (Thaksin's) lawyer said he has Covid."

Thailand has some of the world's strictest royal defamation laws protecting King Maha Vajiralongkorn and his close family, with each charge bringing a potential 15-year prison sentence.

Thaksin's lawyer, Winyat Chatmontree, said he would fight the charges. (AFP)