BANGKOK, Aug 3 (Reuters) - Thailand's house speaker said on Thursday that a parliamentary vote for the next prime minister would be postponed, prolonging a political deadlock that has raised questions about stability since a May general election.

Many voters in the May 14 vote rejected nearly a decade of rule by the military and a military-backed government but the progressive party that won, Move Forward, has been blocked from taking power because of conservative opponents and a nominated upper house.

Parliamentary speaker Wan Muhamad Noor Matha said the vote for a prime minister, which had been scheduled for Friday, could only be held after the Constitutional Court rules on a appeal by Move Forward against the thwarting of its bid to appoint a premier.

"We have to wait for the constitutional court to make its decision on August 16 before determining when we will have the vote again," Wan Noor told reporters.

The Thai baht has weakened this week on the political uncertainty.

Pheu Thai, the latest incarnation of a party founded by former telecommunications tycoon Thaksin Shinawatra, came second in the polls and is hoping to get its candidate elected prime minister to form a government.

After the election Move Forward, along with Pheu Thai and six other parties, forged an alliance to try to form a government. But Move Forward's leader, Pita Limjaroenrat, was twice blocked by parliament from becoming prime minister.

On Wednesday, Pheu Thai said it would seek to form a new alliance without Move Forward and would nominate real estate tycoon Srettha Thavisin for the premiership.

Move Forward won the votes of many young people and its exclusion from power by conservatives allied with the royalist-military establishment has raised the prospect of a return of the sort of street protests that have brought intermittent turmoil to Thailand over the past 20 years.