Pita Limjaroenrat, whose reformist party won the most seats at Thailand’s national elections in May, resigned as its leader on Friday.
The young challenger led the Move Forward Party (MFP) to the top spot in the polls, capitalising on a swell of voters furious at a near-decade of junta-backed rule.
However, he was stopped becoming prime minister by entrenched conservative blocs in parliament, and was later suspended as an MP.
The MFP left a coalition partnership with opposition rivals Pheu Thai, who went on to form a coalition government with pro-military parties, and said they would go into opposition.
“I resigned as the MFP’s party leader to pave the way for an MP that is able to have a voice in parliament, be the opposition leader,” Pita wrote on his official Facebook page.
Under current rules, the leader of the opposition must be an MP.
“Due to my MP suspension, I won’t be able to get my MP position and be opposition leader in the near future,” Pita posted.
In July he was suspended as an MP while waiting for the Constitutional Court’s ruling over his ownership of shares in a now-defunct media company.
MPs are prohibited from owning media shares, under the Thai constitution.
In the Facebook post, Pita said he would remain closely involved in the party.
“No matter what my role is, I will still be involved in MFP and will work closely with the people to my fullest capacity, so that we could achieve something together.”