The vote was scheduled for Thursday following two unsuccessful attempts by the leader of the election-winning Move Forward Party, Pita Limjaroenrat, to become prime minister. Pita faced tough resistance from conservative opponents and appointed senators because of his party's liberal agenda.

"There won't be a meeting on July 27," house speaker Wan Muhamad Noor Matha told reporters at parliament. "I will inform when the next meeting will be held."

Voters rejected nearly 10 years of rule by the military and a military-backed government in the May election with Move Forward winning the most seats. Another opponent of military rule, the populist Pheu Thai party, came second.

But under a constitution drafted during military rule, members of a junta-appointed Senate also vote for the prime minister and Pita failed to win the necessary majority in a joint sitting of both houses.

Pheu Thai, which is backed by the self-exiled former telecoms tycoon Thaksin Shinawatra, was expected to nominate its candidate for premier this week, as part of an eight-party alliance that includes Move Forward.

The postponement was due to a petition to review a decision that blocked the re-nomination of Pita as a prime ministerial candidate on July 19, the parliament's secretary general told lawmakers in a letter, which was seen by Reuters.

"Therefore it is appropriate to cancel the third parliamentary meeting on Thursday 27 July 2023," the letter said.

The independent ombudsman's office petitioned the Constitutional Court to review that decision, saying parliamentary regulation could not trump constitutional rules on nominating a prime minister, following a Move Forward request.

IDEOLOGICAL DIFFERENCES

Pro-democracy protesters have taken to the streets in recent days to denounce the Senate for blocking Pita's path to power. The demonstrations have been peaceful.

Move Forward secured a surprise victory in the May vote, shoring up youth support with progressive promises including ending business monopolies. Its most controversial proposal is an amendment of the lese majeste law, which prohibits any criticism of Thailand's powerful monarchy.

Seven other parties, including the Pheu Thai, are trying to form an alliance with Move Forward, but a meeting of the group scheduled for Tuesday was cancelled. Pheu Thai lawmaker Sutin Klangsaeng said that talks within the group were underway to find a way forward.

Pheu Thai also held talks with other parties over the weekend, including with conservative Bhumjaithai and Palang Pracharat parties, but both said they would not join any coalition with Move Forward due to ideological differences.