The maverick leader of a new Thai political party that finished third in the general election has been stripped of his seat in parliament for violating election law.

Thanathorn Juangroongruangkit, the leader of the Future Forward party and a fierce critic of the Thai government and military, was found guilty of holding shares in a media firm when he applied to be an electoral candidate this year.

The 40-year-old politician was striving to become prime minister and end the military’s political dominance in the country.

Future Forward won 80 seats out of 500 in parliament’s lower house, making it the third most successful party in March’s general election. The party claimed the charge against its leader was politically motivated.

Supporters of Thanathorn, a billionaire entrepreneur, had accused the Thai prime minister, Prayuth Chan-ocha, of setting unfair election rules in order to maintain power. They also accused the courts of being pro-government.

Thailand’s constitutional court found that Thanathorn owned shares in V-Luck Media, in violation of media ownership rules for parliamentary candidates. Thanathorn argued that the firm, which published magazines, stopped functioning as a media company in November 2018, when its employees left their jobs.

“It didn’t generate income from media products and services,” he said on Friday. “The only source of revenue is selling the company’s facilities. How can it be considered media?” He claimed he transferred his V-Luck Media shares to his mother before applying to be an electoral candidate, but on Wednesday the court said this had not been proved.

“From the cradle to the grave, three publications [made by V-Luck Media] neither give me any political advantages nor damaged my rivals,” Thanathorn said.

Thanathorn had been viewed as a maverick young leader whose supporters hoped would lead Thailand into a new era of politics. Various military coups have taken place in the country, most recently in 2014, with Prayuth installed by the junta as prime minister. Prayuth retained his position by leading a pro-junta party coalition after the March election.

Viewed as a thorn in the establishment’s side, Thanathorn, as well as being charged with breaking electoral rules, has faced criminal charges of sedition and computer crime charges.

Future Forward supporters said the charges were an attempt to remove Thanathorn as a political threat. The progressive party, viewed as having the potential to quell the political influence of the Thai royal family and the military, had promised to send the army “back to the barracks”. It had also vowed to reduce political censorship in Thailand.

On Friday, Thanathorn said he would continue to work in politics if he was removed from parliament, and that Future Forward would not disband. “It [the charge] deals only with my MP status,” he said at a press conference.

On Wednesday, he told supporters gathered outside court before the ruling: “The Future Forward party is a journey … all of us will continue fighting and keep moving forward.”