By Chayut Setboonsarng and Panarat Thepgumpanat

BANGKOK (Reuters) -Thai lawmakers will vote for a new premier next week, the house speaker said on Wednesday, after a court rejected a bid by election winner Move Forward to challenge parliament's blocking of its candidate.

The Constitutional Court dismissed the petition by allies of the party on a technicality, arguing its prime ministerial candidate, Pita Limjareonrat, who was twice thwarted by parliament, was not among the complainants.

Thailand has been under a caretaker administration since March and parliament has been deadlocked for weeks over the formation of the next government, after the anti-establishment Move Forward met fierce resistance from conservative lawmakers allied with the royalist military.

Move Forward was the surprise winner of the May election, riding on the back of huge youth and urban support for a liberal policy platform that took aim at some of Thailand's most powerful interest groups, including big business monopolies and the military.

The court's rejection was the latest in a succession of blows for Move Forward. Pita said the party would not give up its fight to govern.

"The problem is at parliament, so we will fix it in parliament," Pita said.

The Pheu Thai party, which was a close second in the election, withdrew its backing for Move Forward after Pita's re-nomination was rejected.

Pheu Thai, the populist electoral juggernaut of the billionaire Shinawatra family, is lobbying aggressively for support from conservative, army-backed lawmakers for its candidate, real estate tycoon Srettha Thavisin.

But it is far from certain whether the politically inexperienced Srettha has enough support from the elected lower house and the military-appointed Senate to become prime minister and form a government.

He needs the backing of half of the combined houses, a tough task given Pheu Thai's bitter history with the military and conservative establishment, which has intervened repeatedly to topple its governments during 17 years of on-off political turmoil.

"Whether the choosing of the prime minister will be completed on the day is up to the session," house speaker Wan Muhamad Noor Matha told reporters in announcing the Aug. 22 vote on the premiership.

Senior Pheu Thai officials have said they are confident Srettha will prevail even without the support of Move Forward, the biggest party in parliament.

(Editing by Martin Petty and Nick Macfie)