The billionaire frontman of Thailand's anti-military bloc was stripped of his MP status on Wednesday over a media shareholding case, a court ruling that could lead to him being banned from politics and jailed for up to 10 years.
Thanathorn Juangroongruangkit and his upstart Future Forward party rode a youth-propelled wave in March elections making it the third largest in the country and alarming the establishment.
The outspoken 40-year-old connected with Thais weary of the army's role in Thailand's turbulent politics, which has seen at least 12 coups since 1932.
But since the vote he has battled legal cases and was suspended from parliament earlier this year after accusations he held shares in a media company when he registered as a candidate.
He denied the charges and said they are politically motivated, but judges at the constitutional court ruled against him on Wednesday, calling his evidence "suspicious" and revoking his MP status.
His removal means the Election Commission could bring criminal charges that carry serious penalties - including a political ban and imprisonment.
It also deals a gut punch to his party, which won devoted fans among millennials after calling for the military to be purged from politics, military conscription to be ended and defence spending slashed.
Thanathorn has repeatedly insisted the 675,000 shares he held in V-Luck Media were sold to his mother on January 8 -- weeks before he registered to run as a candidate.
But the court rejected the claim, saying there was no "official proof" that the transfer took place on that day.
Thanathorn remained defiant following the verdict, telling reporters outside the court that the judges had given "more weight to their presumptions" instead of the evidence he provided.
He assured dozens of his supporters waving the party's signature orange flags that he has not given up and will remain the party leader.
"Future Forward's journey has not yet ended," he said.
- 'He's not the first' -
Thai politics has been marked by short-lived civilian governments, bloody street protests, and coups by an arch-royalist military.
Thanathorn's removal from parliament was a "foregone conclusion" the minute his ultra-popular party made massive strides in the election, said political analyst Thitinan Pongsudhirak of Chulalongkorn University.
"What he and his party stood for is antithetical to the established centres of power in Thailand," he told AFP.
"He's not the first and he won't be the last."
The same court had disbanded a different anti-military opposition party linked to former premier Thaksin Shinawatra right before the March vote.
The poll -- held after nearly five years of junta rule -- saw coup leader Prayut Chan-O-Cha voted in as civilian premier with the help of junta-appointed senators.
But Wednesday's ruling was an indication that despite elections, "Thai authorities are not ready for an open and free democracy," said Charles Santiago, chair of ASEAN parliamentarians for Human Rights, in a statement.
The attempts to silence Future Forward were due to its threat to status quo, he added.
Hours after the court's verdict, the party announced that Thanathorn would join a public campaign to end military conscription at a popular downtown Bangkok mall.
Future Forward has also come under fire for being a rare voice of criticism against Thailand's royalist establishment.
The monarchy, one of the world's richest and headed by King Maha Vajiralongkorn, is at the apex of Thai power.
Last month, 70 Future Forward MPs voted against a royal decree transferring two key army units to the direct control of the king -- an unprecedented political objection to a royal command.
The decree still passed parliament with an overwhelming majority.
Thailand's constitutional court has ruled against Future Forward Party leader Thanathorn Juangroongruangkit
Future Forward Party leader Thanathorn Juangroongruangkit has been a fierce critic of the army's role in Thailand's turbulent politics
Thailand's monarchy, headed by King Maha Vajiralongkorn, is at the apex of power