Among the 44 under investigation are 25 current lawmakers of the People’s Party, Move Forward’s latest incarnation and the largest party in parliament.
Thailand’s lese-majeste law, or article 112 of the criminal code, is among the strictest of its kind in the world and carries punishments of up to 15 years in jail for each perceived insult of the royal family. Critics of the law say it has been misused for political purposes to stifle opposition.
The National Anti-Corruption Commission, which has a broad remit that goes beyond graft, said no charges had yet been filed and not all 44 would be summoned.
"We have started calling relevant individuals to hear the facts," its deputy secretary-general Sarote Phuengrampan told Reuters.
"This step is to collect evidence, but no one has been charged yet."
Under its procedures, if the panel finds sufficient evidence of unethical behaviour, it would then charge people, who can present a defence before a decision is taken on whether to prosecute them in court.
If the Supreme Court finds they committed the offence, they could be banned from politics for life, the same fate suffered last year by a Move Forward politician who made social media posts that were deemed disrespectful to the monarchy.
The latest case was brought by conservative activists in February, two days after the Constitutional Court ordered Move Forward to drop its campaign to change the lese-majeste law.
Move Forward’s anti-establishment policies including military reform and undoing business monopolies, earned it huge urban and youth support, but clashed with powerful interests in Thailand, as demonstrated when lawmakers allied with the royalist military blocked it from forming a government.
Senior People’s Party lawmaker Sirikanya Tansakul said she was preparing a legal defence and was not worried about the threat of a lifetime ban.
"What’s more concerning is that (an unfavourable) decision would set a new precedent: trying to amend can mean a serious ethical violation," she said.
"Amending section 112 or any law would be impossible."