BANGKOK (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.

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

Other charges against the former premier include breaching a computer crime law.Thaksin's lawyer Winyat Chartmontri told Reuters that his client could not appear at Wednesday's hearing due to a COVID-19 infection.

Thaksin, 74, 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. 

The billionaire, who was convicted of abuse of power and conflicts of interest, was released on parole in February after six months in detention.

He made a dramatic return in 2023 from 15 years of self-imposed exile, during which he remained a central figure throughout repeated bouts of political upheaval.

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

The popular opposition Move Forward Party, the biggest in parliament, has found itself in hot water over its campaign to amend the law, with the Constitutional Court due to decide whether to dissolve the party, which would see bans for its leadership.  

Thaksin, the founder of the populist juggernaut Pheu Thai, has seen his family's parties win all but one election since 2001, with three Shinawatra governments toppled by coups or court rulings. 

His return and relatively short time in detention fuelled speculation he had struck a deal with his bitter rivals in the conservative establishment and military which he has long blamed for trying to stifle pro-Thaksin governments. His allies have denied any such deal. 

Pheu Thai leads the current government, with Thaksin's business ally Srettha Thavisin the prime minister and daughter Paetongtarn Shinawatra the party chief. 

Commentators are anticipating Thaksin will seek to wield significant political influence from behind the scenes, raising the possibility of another confrontation with his powerful rivals in the establishment. He insists he has retired from politics.

(Reporting by Panu Wongcha-um, Chayut Setboonsarng and Panarat Thepgumpanat; Editing by Martin Petty)

By Panu Wongcha-um and Panarat Thepgumpanat