Move Forward leader Pita Limjaroenrat announced on Monday the formation of a planned coalition government with a total of 309 MPs and himself as prime minister.
Mr Pita made the announcement after the Election Commission declared the Move Forward Party the winner in Sunday's general election with 151 MPs - 112 from constituencies and 39 from the party list.
He said he took the victory as a mandate from the people for his party to be the core in the formation of the next government.
Mr Pita said he had called Paetongtarn Shinawatra, leader of the Pheu Thai Family, and invited her party to join a coalition.
The three other former opposition parties he had contacted were Thai Sang Thai, Prachachart and Seri Ruam Thai. The five parties had 308 MPs combined.
Mr Pita said he was contacting Pen Tham, or the Fair Party, which had one party-list MP, to join the coalition. He said it was a party which had worked for peace in the three southernmost border provinces.
The six parties would have 309 MPs in total, enough to be a majority government, he said.
"We would form the government as soon as possible, so that there would not be a political and economic vacuum. Please be assured that Move Forward will be fast and meticulous," Mr Pita said.
Asked whether he was concerned about a petition filed against him with the Election Commission over his shares in a defunct media firm, Mr Pita said he was ready to clear this matter with the Election Commission. People should not worry about it, he added.
