Thailand's Constitutional Court is to decide on Tuesday whether to dissolve the opposition Future Forward Party, in a ruling that could strengthen the military-backed government's majority in parliament.

The case is based on a complaint that claims the party is seeking to overthrow the country's constitutional monarchy and is linked to the Illuminati, an alleged secret society that conspiracy theorists believe seeks world domination.

More:

Founded almost two years ago by auto-parts billionaire Thanathorn Juangroongruangkit, Future Forward Party came third in last year's general election, which the opposition says was manipulated in favour of the pro-military Palang Pracharat Party.

Thanathorn, 41, has emerged as the most prominent opponent to the government, which reinstalled former junta leader Prayuth Chan-ocha, 65, as a civilian prime minister five years after he staged a military coup.

Future Forward spokeswoman Pannika Wanich said the case was an attempt to eliminate the party, using a claim of protecting the monarchy that few would dare argue against.

"It's clear that the anti-monarchy claim is the only one that can go so far as to destroy Future Forward Party," Pannika said.

"We emphasise that this is a deliberate political harassment."

The case, accepted by the Constitutional Court in July, is best known for its allegation that the party's triangular logo signifies association with the Illuminati, making the party a threat to Thailand's constitutional monarchy.

'New low'

It also alleges that language in the party's manifesto, as well as past business investments, Facebook pictures and academic briefings by key party figures, suggest personal hostility towards the constitutional monarchy since before the party was founded.

"I looked into their behaviour from the past to now," Nattaporn Toprayoon, who filed the complaint last May, told Reuters. "I did it because I'm Thai. The monarchy is of utmost reverence."

But Pannika said the allegations were a problem.

"This would set a new legal precedent that executive members of a political party must have their thoughts and minds inspected," she said.

"It's also an unexpected new low that a political party has to defend itself to show it's not involved with the Illuminati."

A verdict to dissolve Future Forward on Tuesday is likely, which could prompt political events like flashmobs in the short term, said Yuttaporn Issarachai, a political scientist at Sukhothai Thammathirat Open University.

"But even if the party survives, it will still face other outstanding legal challenges ahead," Yuttaporn said.

Future Forward faces another case alleging that it had violated laws governing political parties by accepting loans from its party leader, among others.