Thailand's parliament on Thursday rejected election winner Pita Limjaroenrat's bid to become prime minister.

After hours of discussions but a surprisingly swift voting process, Pita failed to secure the 375 parliamentary votes needed to become Thailand's 30th premier, despite his reformist party winning the popular vote in the general election.

The political challenger rode a wave of support in May that saw voters emphatically reject almost a decade of army-backed rule under Prayut Chan-o-cha, who took power in a 2014 coup.

But the outcome had become increasingly inevitable, with signs conservative MPs of the lower house and junta-appointed senators would not give him their support.

Only three years ago, massive pro-democracy protests erupted in Bangkok after a court dissolved Pita's Move Forward Party's predecessor, the Future Forward Party.

The path forward remains unclear, with the house speaker yet to indicate if Pita can try again to secure the necessary votes.

Following May's election, he cobbled together an eight-party coalition totalling 312 votes, still short of the needed 375. His bid was sunk by the senators, the vast majority of whom voted against him, which raises questions about whether another round would end in a different result.

MFP's reformist agenda – including suggestions to amend the country's strict royal defamation laws – has drawn objections from conservatives and the country's powerful establishment.

The party's plans to shake up business monopolies are similarly unpopular with the old guard.

Pita's loss may also spell trouble for his coalition, which includes opposition runner-up the Pheu Thai party.

There has been speculation about whether the more established party might now back someone else for the top job. Potential candidates include Pheu Thai leader Paetongtarn Shinawatra, daughter of exiled ex-leader Thaksin Shinawatra, and the party's Srettha Thavisin, a business leader. (AFP)