Thai Prime Minister Prayut Chan-O-Cha dissolved parliament on Monday, setting up a general election in May as the former coup leader seeks to extend army-backed rule.

The vote pits unpopular former army chief Prayut, who came to power in a 2014 putsch, against the daughter of billionaire former PM Thaksin Shinawatra, who still casts a shadow over the kingdom's political scene despite more than a decade in exile.

The main opposition Pheu Thai group, fronted by Paetongtarn Shinawatra, is polling strongly but Thailand's junta-scripted 2017 constitution will make it hard for her to secure the top job.

A statement in the official Royal Gazette published on Monday announced the dissolution, and the Election Commission will confirm the date of the poll later, with May 7 or 14 tipped as most likely.

The election is the second since the 2014 coup and the first since the country was rocked by massive, youth-led pro-democracy protests in Bangkok in 2020.

Unofficial campaigning has been under way for weeks, with rising living costs and the kingdom's sluggish recovery from the coronavirus pandemic high on the agenda.

Prayut, speaking after the announcement, declined to say whether he was satisfied with his tenure, telling reporters: "I cannot answer, you have to ask the people."

The 68-year-old, who cemented his rule in a controversial election in 2019, has demonstrated a longevity rare in Thai politics.

But in a poll published Sunday of who voters would like to see as PM, Prayut lagged in third place on just over 15 percent -- way behind front runner Paetongtarn on 38 percent.

In the same poll of 2,000 people, conducted by the National Institute of Development Administration, nearly 50 percent said they would vote Pheu Thai, with Prayut's United Thai Nation party on around 12 percent.

- Odds favour army -

Pheu Thai has said it is targeting a big victory to prevent the military establishment from blocking its route to power -- avoiding a repeat of 2019 when it won the most seats but was shut out of government.

Under the army-drafted 2017 constitution, the prime minister is chosen by the 500 elected lower-house MPs as well as 250 military-appointed senators.

"I have a strong hope that we can form the government for sure," Paetongtarn told reporters at a rally on Friday.

"That's why we... campaign about the landslide, because the landslide is going to make us strong enough to form the government."

If successful, she would be the third Shinawatra to become premier, after her father and his sister Yingluck -- ousted by Prayut's coup.

The opinion polls would indicate Pheu Thai is in with a chance, with many voters sick of Prayut and the lack of progress they have seen in their own lives.

Prayut is polling behind the pro-reform Move Forward Party, which hopes to capitalise on the anti-establishment spirit of the 2020 street protests.

"I would argue that on the ground for the past four years the sentiment of the era has changed so much for every institution in this country," Move Forward leader Pita Limjaroenrat told AFP, saying he believed voters were ready for change.

"I am sure they will vote for the future, and not the good old days."

- Coup-prone -

Thailand has seen more than a dozen coups since the birth of democracy in 1932, and the military-royalist establishment is a major force.

It remains to be seen whether this elite is prepared to accept another prime minister linked to Thaksin, a bogeyman figure for them.

Chulalongkorn University political analyst Thitinan Pongsudhirak told AFP the election would be the "most consequential in my lifetime".

It will decide whether the kingdom "breaks out of an entrenched, prolonged rut that goes back two decades", he said.

If Pheu Thai misses its landslide, it is possible that two or more military-linked parties -- such as Prayut's United Thai Nation and the ruling Palang Pracharath Party -- would form a coalition.