By Panarat Thepgumpanat and Chayut Setboonsarng
BANGKOK (Reuters) - Thailand dissolved its parliament on Monday to clear the way for an election in May, a vote set to reignite a long-running power struggle between a military-backed establishment and a political movement that has dominated elections for two decades.
King Maha Vajiralongkorn endorsed a decree to dissolve parliament, according to an announcement in the Royal Gazette on Monday, ahead of an election that must be held within 45 to 60 days.
No date has been announced, but two sources with knowledge of the matter earlier on Monday told Reuters the vote would be on May 14.
"This is a return of political decision-making power to the people swiftly to continue democratic government with the King as head of state," said the decree published on Monday.
The election broadly pits the billionaire Shinawatra family and its business allies against parties and politicians close to their rivals among the royalist military and old money conservatives.
With populist policies aimed at Thailand's working classes, parties controlled by the Shinawatras have won every election since 2001, including twice in landslides, but three of its governments were removed in military coups or by court rulings.
The May election will choose members of parliament, which together with an appointed Senate will choose a prime minister by the end of July, according to a timeline provided by the government.
The main opposition Pheu Thai party's Paetongtarn Shinawatra is the frontrunner to be prime minister in opinion surveys, with her support jumping 10 points to 38.2% in a poll released at the weekend, more than twice the backing of her nearest contender.
Incumbent Prayuth Chan-ocha, who has been in power since his coup in 2014 against Paetongtarn's aunt, Yingluck Shinawatra, has been trailing in polls and was third in the latest survey by the National Institute of Development Administration (NIDA).
Prayuth, 68, will continue to lead as head of a caretaker government and is expected to run again.
"I'm glad I've built something good, generated revenue for the country, built industry. There has been a lot of investment," Prayuth said.
"You have to ask the people if they are satisfied or not ... I have done a lot in the many years that have passed."
The NIDA poll of 2,000 people also showed that 50% of respondents would choose candidates from Pheu Thai.
Srettha Thavisin, a property tycoon and senior Pheu Thai adviser who has been campaigning alongside Paetongtarn, said parliament's dissolution was a "turning point" for Thailand.
"I would like to invite everyone to study policies, positions and ideologies of all parties to find the one for you that will make the country of your dreams come true, to move the nation forward and improve lives," he said on Twitter.
Paetongtarn on Friday said she was confident of winning by a landslide, with the aim of averting any political manoeuvring against her party.