Paetongtarn Shinawatra won the backing of parliament to become Thailand's youngest prime minister on Friday, only a day after she was thrust into the spotlight amid an unrelenting power struggle between the warring elites.

The 37-year-old daughter of divisive political heavyweight Thaksin Shinawatra won easily with 319 votes, or nearly two-thirds of the house.

She was not present in parliament and watched the vote from her Pheu Thai Party's headquarters.

Paetongtarn now faces a baptism of fire, just two days after ally Srettha Thavisin was dismissed as premier by a judiciary central to Thailand's two decades of intermittent turmoil.

At stake could be the legacy and political future of the billionaire Shinawatra family, whose once unstoppable populist juggernaut suffered its first election defeat in over two decades last year, and had to do a deal with its bitter enemies in the military to form a government.

Paetongtarn will immediately face challenges on multiple fronts, with the economy floundering and the popularity of her Pheu Thai Party dwindling, having yet to deliver on its flagship cash handout programme worth 500 billion baht.

Paetongtarn has never served in government and will become Thailand's second female prime minister and the third Shinawatra to take the top job after aunt Yingluck Shinawatra, and 75-year-old father Thaksin, the country's most influential and polarising politician. (Reuters)