Thailand's parliament confirmed on Friday it would vote again for a new prime minister next week, after military-appointed lawmakers foiled liberal frontrunner Pita Limjaroenrat's bid for the top job.

Pita's Move Forward Party (MFP) won the most seats in the May elections, buoyed by young Thais eager for progressive reforms after nine years of army-backed rule in the kingdom.

But the Harvard-educated millionaire's campaign to lead the next government was knocked back on Thursday by the legislature's senators, who consider his pledge to reform strict royal defamation laws a red line.

And the vote came just a day after Thailand's top election body recommended the Constitutional Court suspend Pita as an MP – providing more fuel for senators already poised to vote against him.

Parliament will hold its second ballot for a new prime minister on July 19, deputy house speaker Pichet Chuamuangphan told AFP news agency on Friday.

But it remained unclear whether Pita would be renominated or if he would face any additional candidates.

"We still have to talk about it first," said Pichet, who is a member of MFP's coalition partner Pheu Thai. "For now, it's gathering opinions and feedback from the people."

Pita has insisted that he remains a candidate to lead the next government and was "not giving up" despite taking only 324 votes overall – well short of the 375 he needed.

Just 13 senators voted for him, with many voicing their opposition to MFP's pledge to soften the royal defamation laws.

Experts are expecting Pheu Thai – a party that held office before the 2014 military coup but is now in an uneasy alliance with MFP – to field prime ministerial candidates of their own.

Pheu Thai secretary-general Prasert Jantararuangtong has told reporters that the party would meet with MFP on Friday evening to discuss the strategy for the next ballot. (AFP)