BANGKOK, May 29 (Reuters) - Thailand's attorney-general will indict former Prime Minister Thaksin Shinawatra for allegedly insulting the monarchy, an official said on Wednesday, in a setback to a political heavyweight whose loyalists are currently in government.

The complaint, lodged by the royalist military that ousted the government of his sister, Yingluck Shinawatra, had stemmed from an interview the influential tycoon gave to foreign media in 2015.

Other charges against him include violating a computer crime law.

"The attorney-general has decided to indict Thaksin on all charges," spokesperson Prayuth Bejraguna said, adding the former premier would need to appear before court on June 18.

Thaksin, 74, denies wrongdoing and has repeatedly pledged his loyalty to the crown, criticism of which is forbidden under Thailand's lese-majeste law, which is one of the world's strictest of its kind.

Thaksin would be the highest-profile case among more than 270 prosecutions in recent years under the controversial law, which carries a maximum jail term of 15 years for each perceived insult of the royal family.

His lawyer Winyat Chartmontri said a comprehensive defence had been prepared and his client would seek bail. He also questioned the authenticity of the video of the interview in which the alleged insult was made.

Thaksin did not appear at Wednesday's hearing because he had been infected with COVID-19, he said.

"We have to proceed based on our rights in a criminal case," Winyat told a press conference.

"Thaksin Shinawatra is ready to prove his innocence in the justice system."