People's Party (PP) secretary-general Sarayut Jailak said on Monday he would resign after the party's election performance fell below expectations.
Mr Sarayut told reporters that the party had suffered some setbacks and lost in several constituencies in the election, adding that he had previously discussed within the party his intention to step down from his role over the results.
He said that such a decision was also a consideration he had before the polls on Sunday.
He said he will officially announce his resignation during the party's next big meeting, which will be held in two months.
Asked whether the PP's loss stemmed from its decision to vote for Anutin Charnvirakul as prime minister before declining to join the government, Mr Sarayut said this was not the main factor, noting that the results were mixed rather than a total defeat.
"We win in all of Bangkok. We also lost in some of the constituencies we had won because Bhumjaithai has been a stronger party," he said.
Mr Sarayut said his resignation doesn't mean he has lost hope in the People's Party, but that he wants to create an opportunity for new people to fix the party and turn its weaknesses into strengths.
"I've worked behind the scenes with the party for a long time and have had considerable influence," Mr Sarayut said.
"I believe it's time to allow people with new ideas to step in."
Regarding his future role, he said that, while he may no longer serve in party management, he could continue working at the provincial level or in other capacities.
He said he will remain an active party member and will wait for internal discussions to conclude before deciding his next step.
On the issue of a National Anti-Corruption Commission meeting on Monday to consider allegations against 44 former Move Forward Party MPs over ethical violations related to efforts to amend Section 112 of the Criminal Code, Mr Sarayut said the party had already assessed the risks.
The case involves three PP constituency MPs and 12 party-list MPs, and the party would wait for the final ruling before taking further action.
